GB2165234A

GB2165234A - Methods of preparing doped silica glass

Info

Publication number: GB2165234A
Application number: GB08524074A
Authority: GB
Inventors: Masanobu Motoki; Nobutaka Matsuo; Sadao Kanbe; Haruo Osafune; Yoshitaka Itoh
Original assignee: Suwa Seikosha KK
Current assignee: Suwa Seikosha KK
Priority date: 1984-10-05
Filing date: 1985-09-30
Publication date: 1986-04-09
Also published as: GB8524074D0; AU589577B2; AU4832385A; GB2165234B

Abstract

A method of preparing doped silica glass comprises the steps of preparing a sol solution by mixing a hydrolysed solution and a solution including ultra fine particles of silica, gelling the sol solution to produce a gel, drying the gel to produce a dry gel, sintering the dry gel to produce silica glass, and adding a dopant to one or more of the hydrolysed solution, the solution of ultra-fine particles of silica, the sol solution, the gel or during drying or sintering.

Description

SPECIFICATION Methods of preparing doped silica glass This invention relates to methods of preparing doped silica glass for use, for example, in the manufacture of optical fibres.

A known sol-gel method of preparing doped silica glass has the following advantages: (1) Glass is obtained with relatively small energy consumption; (2) Glass is easily formed from the components which do not constitute a uniform glass in other conventional methods; (3) Glass of high purity is obtained; (4) Glass in which various components are included uniformly is obtained.

Kamiya and his co-inventors developed a TiO2 - SiO2 glass having a smaller thermal expansion coefficient than silica glass for use in astronomical telescopes. (Japan Chemics. Conference Bulletin, No. 10, 1571 1981).

Sato and his colleagues introduced the method of preparing silica gels including additives such as Ge, Ti, Zr, Ta, Nb, Sb, etc. which serve as parent materials for optical glass for various applications. (Japanese Laid-Open Publication No. 57/191221).

However, the conventional methods of preparing silica glass have disadvantages. The Kamiya et al method takes more than four months to complete and TiO2 - SiO2 glass of acceptably large size has not yet been obtained. In Sato et al's method, the size of the obtained dry gel is about 4mm in diameter and 50mm in length which cannot result in a doped silica glass of practically acceptable size (for example, a substrate 6 inches square or a rod of 20mm in diameter and 500mm in length.

Another sol-gel method for preparing cylindrical doped silica glass is disclosed in Japanese Laid-Open Publication No. 56/104732. However, this method has the disadvantage that a cylindrical doped silica glass of high purity without fractures is difficult to obtain.

Thus the present invention seeks to provide a method of preparing silica glass in quantities which are large enough for industrial use and which are of high purity and which are produced with good yield.

Although the present invention is primarily directed to any integer or step, or combination of integers or steps, herein disclosed, nevertheless, according to one particular aspect of the present invention to which, however, the invention is in no way restricted, there is provided a method of preparing doped silica glass comprising the steps of: preparing a sol solution by mixing a hydrolysed solution and a solution including ultra fine particles of silica; gelling the sol solution of produce a gel; drying the gel to produce a dry gel; sintering the dry gel to produce silica glass; and adding a dopant to one or more of the hydrolyzed solution, the solution of ultra-fine particles of silica, the sol solution, the gel or during drying or sintering.

The dopants used in the present invention may be Li, Na, K, Cs, B, Al, Ga, Ge, N, P, F, Zr, Ti, Ta, TI, Pb, Ag, etc. but is not limited thereto.

Preferably the hydrolysed solution comprises a solution of any alkyl silicate.

In preparing larger dry gels the container for the gel is preferably hydrophobic and made of an organic polymer such as polypropylene, polyethylene fluoride, polyvinyl chloride, polyethylene, polystyrene, etc., or a material consisting of an inorganic material such as glass, coated with an organic polymer such as the above, are most suitable.

In preparing cylindrical silica glass for use in the manufacture of optical fibres, germanium alkoxide, which increases the refractive index of the glass, is desirably used as the dopant.

The yield of the transparent glass is enhanced by adding a solution of ultra fine particles of silica. The solution of ultra fine particles of silica may be prepared by hydrolysing an alkyl silicate with aqueous ammonia or ammonia gas and water. However, since ammonia is used, if the solution of ultra fine particles of silica is directly added to the hydrolysed solution, the sol solution gels abruptly, which is not desirable. Accordingly, the pH of the solution of ultra fine particles of silica should be adjusted to less than 7.0 before addition of the hydroylsed solution. Thus the method may include the step of adjusting the pH of the sol solution to a predetermined value by adding ammonia, aqueous ammonia or an organic base. The organic base may be one or more of triethylamine, pyridine or aniline.

If the silica of the solution of ultra fine particles of silica is less than 20% of the silica in the hydrolysed solution of alkyl silicate, foam is likely to remain in the glass at the time of sintering, which causes fracture of the glass.

In one embodiment the hydrolysed solution is prepared by partially hydrolysing the alkyl silicate with water in a molar ratio between 1 and 3 with respect to the alkyl silicate at a temperature below 20"C, adding the dopant and completing hydrolysation. If water is added in a molar ratio greater than 3%, excess water directly acts with the metal alkoxide, thereby preventing the dry gel becoming transparent in the sintering step as well as making the distribution of the refractive index non-uniform. In the absence of alcoholic solvent, gelation occurs rapidly and yield is lowered so that the hydrolysed solution should be kept at a temperature lower than 20"C when added to the solution of ultra fine particles of silica.

The alternative is where the hydrolysed solution is prepared by adding water to a solution of the alkyl silicate and alcohol at a volume ratio of more than 20% with respect to the alkyl silicate, said water being in a molar ratio of between 1 and 3 with respect to the alkyl silicate, to carry out partial hydrolysation, adding the dopant, and completing hydrolysation.

The mean diameter of the particles of ultra fine silica is adjusted by controlling the amount of ammonia or the amount of alcohol. If the mean diameter of the ultra fine particles of silica is too small, foam remains in the dry gel during the sintering step and if the mean particle diameter is too large fractures are likely to occur during the drying step. Preferably the mean particle diameter of the ultra fine particles of silica is between 0.01 Fm and 1 .0 > m. As the mean particle diameter of the ultra fine particles of silica increases, the dispersion of silica particles deteriorates. Thus it may be necessary that the ultra fine particles of silica are dispersed by ultrasonic vibration and/or centrifugal separation.

In gelling the sol solution, 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 operational variations into one by controlling the gelling time. Through experiments it has been found preferable that the composition of the sol solution is such that the volume of the silica glass obtained by drying and sintering the wet gel is between 5 and 15% of the volume of the wet gel. If the volume ratio of the wet gel and a resulting transparent glass obtained by drying and sintering the wet gel is too large, a long time is required for drying and fracture is likely to occur when shrinking. If the volume ratio is too small, the dry gel easily fractures during drying.

The step of drying the wet to the dry gel has a great influence on yield on silica glass and is an important step. Experiments indicate that it is desirable to dry a wet gel to a dry gel with good yield in a short period. Thus the wet gel may be dried in a cylindrical container having lids at each end, each lid having openings amounting to less than 15% of the surface area thereof. Alternatively, the wet gel may be removed from a cylindrical container and dried in a further container having openings amounting to less than 15% of the surface area thereof. The wet gel may be dried at at temperature between 0 C and 1 00 C and heated to a temperature between 20"C and 1200C at a rage of less than 1200C per hour.

Preferably the dry gel is sintered by removing absorbed water, removing carbon, removing hydroxide groups, removing chloride and/or fluoride ions, and making the dry gel non-porous.

The removal of the absorbed water has a great influence on yield. If the gel is abruptly heated, the occurrence of fractures increases, thereby lowering the yield. If the heating rate is too slow, a long time is required for the process and manufacturing costs increase. Experiments demonstrate that the upper limit of acceptable heating rate which does not lower the yield is about 400"C per hour. Preferably the absorbed water is removed by heating the dry gel to a selected temperature between 20"C and 400"C at a rate less than 400"C per hour and maintaining the dry gel at the selected temperature for at least one hour.Carbon may be removed by heating the dry gel to a selected temperature between 400"C and 1200"C at a rate between 30"C and 400"C per hour and maintaining the dry gel at the selected temperature for at least one hour. In addition to the removal of carbon, hydrochloride ions are decomposed and a dehydrating condensation reaction is accelerated, thereby reducing the remaining hydroxide groups.

This step also has an influence on yield.

If hydroxide groups exist in the doped silica glass, light whose wavelengths are about 1.95 > m is absorbed which causes inferior optical properties in the resulting optical fibres. From experiment it is preferred that the hydroxide groups are removed by heating the dry gel to between 700"C and 1200"C in a stream of a carrier gas of helium, neon, argon, nitrogen or oxygen or mixtures thereof and a hydroxide group removing agent in a ratio between 1 and 40% with respect to the carrier gas, in a sintering oven.

The hydroxide group removing agent may be Cl2, SOCI, SF6, CF4, C2F6 or C3F6.

The purpose of removing chloride and/or fluoride ions is to prevent the hydroxide removing agent used in the preceding step from remaining in the glass which causes foaming. Thus, after removing the hydroxide groups, chloride andlor fluoride ions may be removed by heating the dry gel to between 700"C and 1200"C in an oven in a stream of a carrier gas of helium, neon, argon or nitrogen or mixtures thereof. It is desirable to make the dry gel non-porous by heating the dry gel to between 1000"C and 1500"C in a stream of helium gas.

Preferably the method includes the step of, after making the dry gel non-porous, the dry gel is heated to between 1000"C and 1600"C and maintained at a selected temperature for a given period of time to produce a transparent silica glass. Preferably the heating rate during the removal of hydroxide groups, chloride and/or fluoride ions and making the dry gel non-porous, is between 0.5 and 1000"C per minute.

The invention will now be described in detail with reference to the following Examples.

Example 1 80ml of 0.02 normal hydrochloric acid was added to 6249 of refined commercially available silicon ethoxide to carry out partial hydrolysis, the temperature of the solution being maintained at 50C in order to prevent abupt gelation of the hydrolysed solution. Still maintaining the temperature at 5"C, 579 of tetra ethoxy germanium were added to the solution and after violent stirring, 152ml of water was added to complete the hydrolysis.

Separately from the above, a mixture of 180ml of 28% aqueous ammonia, 1.8e of ethanol and 325ml of water were added to a mixture of 1t of silicon ethoxide and 1.7e of ethanol and the solution was stirred at room temperature and allowed to stand overnight. Then the resulting solution was condensed to 720ml to obtain a solution including ultra fine particles of silica whose mean diameter was 0.17cm. The pH of the solution was adjusted to 5.0 by the addition of 2 normal hydrochloric acid in order to avoid abrupt gelation when the solution was added to the hydrolysed solution prepared as above.

The hydrolysed solution and the solution including ultra fine particles of silica were mixed to yield a sol in which the ratio of the effective glass component of the hydrolysed solution and that of the solution including ultra fine particles of silica was 4 to 6. The pH of the sol was then adjusted to 3.5 by adding 0.2 normal aqueous ammonia and the volume was adjusted to 1700ml by adding water.

The sol, adjusted as above, was fed into a cylindrical container of polyethylene (inner diameter 6.0cm, length 75.0cm) to about 80% of the capacity of the container (i.e. 60cm) and the container was sealed.

After 25 minutes at room temperature (300C) the sol was gel led. The resulting gel was maintianed in the cylindrical container overnight and the cover of the container was exchanged for a cover having openings amounting to 1.0% of the surface area. Then the gel was heated from 30 to 60"C at 20"C per hour and dried. Thus a dry gel (4cm in outer diameter and 41cm in length) which did not fracture, even at room temperature, was obtained.

4 out of 20 gels formed in the above manner fractured and 16 perfect dry gels were obtained. The yield was thus 80%.

The 16 dry gels were placed in a sintering oven and heated to 200"C at a rate of 30"C per hour and were maintained at 200 C for 1 hour to remove absorbed water. The gels were further heated to 450 C at 30 C per hour, maintained at 4500C for 5 hours, heated to 9500C at 30"C per hour and maintained at 950"C for 1 hour to remove carbon.

The gels were next cooled to 770 C at 5"C per minute at which temperature a stream consisting of a mixture of helium an chlorine in the ratio of 5 to 1 flowed into the oven for 2 hours. Then the gels were heated to 900"C at 5"C per minute and were maintained at 900 C for 1 hour to remove hydroxide groups.

The temperature was raised to 1000"C at 1"C per minute and a gaseous stream of helium and oxygen in the ratio of 3 to 1 flowed into the oven for 1 hour to remove chloride ions. The gels were heated to 1200"C at 1"C per minute in a stream of helium gas to make the gels non-porous and were maintained at 1200"C for 1.5 hours to make the gels transparent to yield cylindrical doped silica glass of high purity (3cm in diameter and 30cm in length). The degree of shrinkage from the wet gel to the resulting glass was 10.5% and the germanium content of the glass was 3 mol% with respect to silica.

In the above sintering process, no dry gels fractured. Accordingly, the total yield in this Example was 80% and 16 complete samples of cylindrical doped silica glass of high purity were obtained.

Example 2 54 ml of 0.01 normal hydrochloric acid and 105.29 of ethanol were added to 6249 of refined commercially available silicon ethoxide and the solution was violently stirred to carry out partial hydrolysis. 939 of tetra isopropoxy germanium was added to the solution and violently stirred and then 184ml of water was added to complete the hydrolysis.

Separately from the above, a mixture of 120ml of 28% aqueous ammonia, 1.5t of ethanol and 216ml of water was added to a mixture of 444ml of silicon methoxide and 14 of ethanol with stirring. The resulting solution was allowed to stand for 1 day and was condensed under reduced pressure to a predetermined concentration to yield a solution including ultra fine particles of silica. The pH of the solution including ultra fine particles of silica was adjusted to 3.0.

The hydrolysed solution and the solution including ultra fine particles of silica were mixed and stirred and impurities removed by centrifugal separation. The pH of the resulting solution was adjusted to 3.2 by bubbling ammonia gas therethrough to obtain a sol solution. The sol solution was fed into a cylindrical container of polyvinyl chloride (inner diameter 4.0 cm, length 140.0cm) to a height of 130cm. At room temperature of 25"C the sol gelled in 20 minutes. The gel was maintained for 2 days in the cylindrical container and then was placed into 100 cylindical spaces (1 cm in outside diameter and 60cm in length) of polyethylene fluoride provided in a drying container of polypropylene having a cover having openings amounting to 0.5% of the surface area.The gels in the drying container were heated from 25"C to 600C at 5"C per hour and were maintained for 8 days, thus producing a dry gel.

5 out of 20 dry gels formed in the same manner as above fractured and 15 complete dry gels were obtained with a yield of 75%.

The above 15 dry gels were placed in a sintering oven and heated to 300 C at 60 C per hour and maintained at 300 C for 5 hours to remove absorbed water. Then the gels were heated to 950 C at 1800C per hour and maintained at 9500C for 18 hours to remove carbon. Then the gels were cooled to 770"C and a gaseous mixture of helium and SOCt2 in a ratio of 5 to 2 flowed in the oven for 2 hours to remove hydroxide groups. The gels were then heated to 12200C at 1"C per minute in a stream of helium gas and maintained at 12200C for 1.5 hours to yield transparent glass (2cm in outer diameter and 60cm in length).

In the above sintering process, 3 out of 15 dry gels fractured and 12 complete samples of cylindrical doped silica glass were obtained with a yield of 80%. Accordingly, in this Example samples of cylindrical doped silica glass of high purity including germanium at 5 mol% with respect to silica were obtained with a yield of 60%.

Example 3 32ml of 0.01 normal hydrochloric acid was added to 12489 of refined commercially available silicon ethoxide and the solution was violently stirred to carry out hydrolysis.

A mixture of 540ml of 28% aqueous ammonia, 5.25e of ethanol and 1e of water was added to a mixture of 3t of silicon ethoxide and 5.25 of ethanol and the solution was stirred and allowed to stand overnight. The resulting solution was condensed under reduced pressure to a predetermined concentration and the pH of the solution was adjusted to 6.0 to yield a solution including ultra fine particles of silica.

The hydrolysis solution and the solution including ultra fine particles of silica were mixed and ultrasonic vibrations applied to the solution to disperse uniformly the silica particles. The pH of the resulting solution was adjusted to 6.0 by the addition of 0.1 normal ammonia to yield a sol solution.

At 40"C, the sol solution was fed into a cylindrical container of polyethylene (inner diameter 4.0cm, length 180cm) to a height of 160cm and the container was sealed. The sol gelled in 15 minutes. After being maintained overnight in the cylindrical container, the cover of the cylindrical container was exchanged for a cover having openings amounting to 0.5% of the surface area of the cover and the gel was heated to 80"C at 100"C per hour. The gel was maintained at 80"C for 7 days and a dry gel (2.80cm in outer diameter and 110cm in length) which did not fracture even at room temperature was obtained.

1 out of 12 dry gels formed in the same way as above fractured and 11 complete dry gels were obtained with a yield of 92%. The dry gels were heated in an oven to 1500C at 60"C per hour, maintained at 150 C for 1 hour, heated to 4000C at 70"C per hour and maintained at 400"C for 1 hour to remove absorbed water.

Then the gels were heated to 950"C at 90"C per hour and maintained at 950"C for 5 hours to remove carbon. Then a gaseous mixture of neon and SF6 in the ratio of 40% with respect to neon, flowed into the oven for 1.5 hours to remove hydroxide groups. The gels were further heated to 10000C at 5"C per minute and the gaseous mixture of neon and oxygen in the ratio of 100% with respect to neon flowed in the oven for 2 hours to remove fluoride groups. Then neon gas flowed into the oven for 2 hours and the gels were made non-porous. The gels were then heated to 1200"C at 6 C per minute for 3.5 hours to yield transparent glass.

In the above sintering process, there were no fractures of the 11 dry gels and thus complete samples of cylindrical pure silica glass (2cm in outer diameter and 83cm in length) were obtained with a yield of 100%.

Example 4 108ml of 0.01 normal hydrochloric acid and 21049 of ethanol were added to 1248g of refined commercially available silicon ethoxide and the solution was violently stirred to carry out partial hydrolysis. Then 1149 of tetra ethoxy germanium was added and the solution was violently stirred and then 368ml of water was added to complete the hydrolysis.

A mixture of 460ml of 28% aqueous ammonia, 3.5e of ethanol and 650ml of water were added to a mixture of 2t of silicon ethoxide and 3.5t of ethanol and the solution was stirred at room temperature and allowed to stand overnight. The solution was condensed to a predetermined concentration and the pH was adjusted to 5.0 to yield a solution including ultra fine particles of silica. The resulting solution including ultra fine particles of silica and the hydrolysed solution were mixed and the pH was adjusted to yield a sol solution. The sol solution was fed into a cylindrical container of polyvinyl chloride (inner diameter 4cm, length 220cm). Thus a wet gel of 4cm in outer diameter and 200cm in length was obtained.

The wet gel was placed in a container having openings amounting to 0.2% of the surface area and dried. Then the gel was heated to 60"C at 100"C per hour and maintained at 60"C for 7 days. Thus a dry gel which did not fracture, even at room temperature, was obtained.

9 out of 20 dry gels formed under the same conditions as above fractured and 11 complete dry gels were obtained with a yield of 55%.

The dry gels were heated to 1500C in an oven at 60"C per hour, maintained at 1500C for 1 hour, heated to 400"C at 60"C per hour, and maintained at 400"C for 1 hour to remove absorbed water and OH groups.

The dry gels were further heated to 900"C at 90"C per hour and maintained at 900"C for 5 hours to remove carbon. Then a gaseous mixture of helium and chlorine in the ratio of 40% with respect to helium flowed into the oven for 1.5 hours to remove hydroxide groups. Then the gels were heated to 1000 C at 5"C per minute and maintained at 1000"C for 3 hours in a gaseous mixture of helium and oxygen in the ratio of 55% with respect to helium to remove chloride ions and to make the gels non-porous.

Thereafter, the gels were heate to 1150"C at 100C per minute and maintained at 1150"C for 5 hours to make the gels transparent. Thus samples of doped silica glass of 2cm in outer diameter and 1000cm in length were obtained.

In the above sintering process, 2 dry gels fractured and in the whole of the Example, 9 complete sam ples of doped silica glass were obtained with a yield of 45%.

Example 5 432ml of 0.02 normal hydrochloric acid were added to 1248g of refined commercially available silicon ethoxide and the solution was violently stirred at room temperature.

A mixture of 360ml of 28% aqueous ammonia, 3.5t of ethanol and 650ml of water was added to a mixture of 2t of silicon ethoxide and 3.5 of ethanol and the solution was stirred at room temperature.

After allowing to stand overnight, the solution was condensed to a predetermined concentration and the pH thereof was adjusted to 5.0 to yield a solution including ultra fine particles of silica.

The hydrolysed solution and the solution including ultra fine particles of silica were mixed to yield a sol solution. The sol solution was fed into a container of polyvinyl chloride (inner diameter 0.5cm, length 200cm) and gelled to a wet gel of 5cm in outer diamter and 170cm in length. The wet gel was placed in a drying container of polypropylene having openings amounting to 0.45% and then the same drying and sintering steps as in Example 1 were followed. Thus pure silica glass of 3cm in outer diameter and 94cm in length was obtained.

3 out of 20 dry gels formed under the same conditions fractured before sintering and 17 complete samples of pure silica glass of high purity were obtained with a yield of 85%.

Example 6 162ml of 0.03 normal hydrochloric acid was added to 9139 of refined commercially available silicon methoxide and the solution was violently stirred at 0 C. Then 499 of tetra methoxy germanium was added with violent stirring and then 288ml of water was added to complete hydrolysis.

A mixture of 110ml of 28% aqueous ammonia, 1.2 of ethanol and 288ml of water was added to a mixture of 592ml of silicon methoxide and 1.2t of methanol and the mixture was condensed to a predetermined concentration. The pH was adjusted to 5.0 to yield a solution including ultra fine particles of silica.

In the same way as in Example 1 using the above solutions and a container of polyethylene, a wet gel of 4cm in outside 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 Example 1, a silica glass was obtained.

2 out of 20 samples fractured and 18 complete samples of doped silica glass including germanium in an amount of 3 mol% with respect to the effective glass component were obtained with a yield of 90%.

Example 7 324ml of 0.01 normal hydrochloric acid was added to 1248g of refined commercially available silicon ethoxide and the solution was sufficiently stirred. Then 114g of tetra ethoxy germanium were added with stirring and finally 108ml of water were added to complete the hydrolysis.

A mixture of 180ml of 28% aqueous ammonia, 1.8 of ethanol and 325ml of water were added to a mixture of 1 of tetra ethoxy germanium and 1.7 of ethanol with stirring. The resulting solution was condensed to a predetermined concentration and the pH was adjusted. To the resulting solution was added the hydrolysed solution to adjust the pH. The sol was gelled in a cylindrical container of polypropylene of 6cm inner diameter and 100cm in length to a wet gel whose outer diameter was 6cm and whose length was 75cm. The wet gel was sintered under the same conditions as in Example 1 and a transparent glass was obtained.

2 out of 20 samples of transparent glass formed in the same way as above fractured and 18 complete samples of doped silica glass were obtained with a yield of 90%.

However, the distribution of germanium in the core portion of the obtained doped silica glass was nonuniform and the quality of the resulting glass was not very good.

Reference Example 108ml of 0.01 normal hydrochloric acid were added to 12489 of refined commercially available silicon ethoxide and the solution was stirred at room temperature without adjusting the temperature. Then 579 of tetra ethoxy germanium were added and the solution was sufficiently stirred. Thereafter 324ml of water was added to the solution with stirring and the solution gelled during the stirring.

Example 8 80ml of 0.02 normal hydrochloric acid was added to 6249 of refined silicon ethoxide and the solution was violently stirred keeping the temperature at 50C to carry out partial hydrolysis. Then 57g of tetra ethoxy germanium was added with violent stirring and then 152ml of water was added to complete hydrolysis.

A mixture of 120ml of 28% aqueous ammonia and 1.8 of ethanol and 325ml of water were added to a mixture of 1 of silicon ethoxide and 1.7e of ethanol and the solution was stirred at room temperature.

After allowing to stand overnight, the solution was condensed to a predetermined concentration. Under the same conditions as in Example 1, a transparent glass was formed. 10 out of 20 samples of glass formed in the same way as above fractured and 10 complete samples of doped silica glass were obtained with a yield of 50%.

Example 9 80ml of 0.02 normal hydrochloric acid were added to 6249 of refined commercially available silicon ethoxide and the solution was violently stirred keeping the temperature at 5"C to carry out partial hydrolysis. The 57g of germanium ethoxide were added to complete the substitution reaction. Finally, 152ml of water was added to complete hydrolysis.

A mixture of 180ml of 28% aqueous ammonia, 1.8 of ethanol and 325ml of water was added to a mixture of 14 of silicon ethoxide and 1.74 of ethanol and the solution was stirred at room temperature.

After allowing to stand overnight, the solution was condensed under reduced pressure to a predetermined concentration and the pH was adjusted to 5.0 to yield a solution including ultra fine particles of silica.

The hydrolysed solution and the solution including the ultra fine particles of silica were mixed and masses were removed by centrifugal separation. Then by following the same drying and sintering steps as in Example 1, a doped silica glass of high quality was obtained. 2 out of 20 samples of doped silica glass formed under the same conditions as above fractured and 18 complete samples of cylindrical doped silica glass were obtained with a yield of 90%.

Example 10 83.69 of 0.02 normal hydrochloric acid were added to 597.69 of refined ethyl silicate to carry out partial hydrolysis, keeping the temperature of the solution at 5"C in order to prevent abrupt gellation of the hydrolysed solution. At 5"C, 51.99 of tetra ethoxy germanium was added to the resulting solution and the solution was violently stirred. 139.29 of 0.02 normal hydrochloric acid were added to complete the hydrolysis. 222.49 of water was added to the solution in order to lower the viscosity thereof and the resulting solution was referred to as a hydrolysed solution.

A mixture of 51 ml of 29% aqueous ammonia, 839ml of ethanol and 271.79 of water were added to a mixture of 785.39 of ethyl silicate and 839ml of ethanol and the solution was stirred at 200C to yield fine particles of silica. After allowing to stand overnight, the solution was condensed under reduced pressure.

Then, in order to enhance the yield at the drying step, alcohol in the condensed solution was substituted by water. The pH of the solution was adjusted to 4.5 by the addition of 2 normal hydrochloric acid to prevent the abrupt gelation when it was mixed with the hydrolysed solution. The resulting solution included fine particles of silica whose mean particle diameter was 0.18 > m and was referred to as the silica dispersed solution.

The silica dispersed solution and the hydrolysed solution were mixed to yield a sol solution and the pH of the sol solution was adjusted to 4.20 by the addition of 0.2 normal aqueous ammonia and water and the volume was adjusted to 1872ml. The sol solution was fed into a cylindrical container of polytetrafluoroethylene (TEFLON) Trade Mark)) whose inner diameter was 50mm and whose length was 1000mm, to a height of 900mm and was gelled in 20 minutes at room temperature of about 20"C.

10 wet gels formed in the same way as above were aged for 3 days and placed in a drying container having openings amounting to 0.2% of the surface area. Then the wet gels were dried at a temperature of 60 C. After 13 days, 10 gels which did not fracture, even at room temperature, were obtained with a yield of 100%. The bulk density of the obtained dry gels was 0.67g/cm3.

The 10 dry gels were placed in a sintering oven and heated to 200"C at 300C per hour, maintained at 200"C for 5 hours, heated to 300 C at 30 C per hour and maintained at 300 C for 5 hours to remove absorbed water. Then the gels were heated from 300"C to 950"C at 609C per hour and maintained at 950 C for 2 hours to remove carbon and ammonium chloride and to acclerate the dehydrating condensation reaction. Thereafter, the gels were cooled down to 8000C and maintained at 800"C for 30 minutes in a stream of helium and chlorine flowing in the ratio of 2/mien and 0.2l/min, respectively.The gels were again heated to 900"C at 60"C per hour, maintained at 900"C for 1 hour, heated to 1000"C at 60"C per hour and maintained at 1000"C for 3 hours to remove OH groups. In a stream of oxygen flowing at 1l/min, the gels were heated to 1100 C at 60"C per hour and maintained at 1100 C for 30 hours to remove chloride ions. Then in a stream of helium gas, the gels were heated to 12500C at 30"C per hour and maintained at 1250"C for 30 minutes to ciose pores in the gel.The gels were further heated from 1250"C to 1400"C at 60"C per hour and maintained at 1400"C for 1 hour to make the gels non-porous. 10 samples of cylindrical transparent glass were obtained in the above manner without any fractures, the yield being 100%.

The dimensions of the samples of transparent glass obtained were 23.2mm in outer diameter and 417.4mm in length and the yield proved to be about 100%.

Moreover, germanium included in the obtained transparent glass was measured by IMA, XMA, ICP, etc., and 3 mol% of germanium was detected throughout the glass which indicated that the yield of germanium was almost 100% and germanium was uniformly dispersed throughout the glass.

Furthermore, the transmission loss of optical fibres of 200 > m in outer diameter obtained by fibre-drawing the transparent glass of this example was less than 4dB/km when the wave length was 1.57 > m which indicated that optical fibres of very low transmittance loss were obtained.

As in this embodiment, by adding germanium to the hydrolyzed solution, high quality doped silica glass was provided.

Example 77 TABLE 1 hydrolysed solution silica particle dispersed solution ethyl silicate 493.2g ethyl silicate 785.3g ethanol 211ml ethanol 839ml 0.2N hydrochloric acid 64.0g water 271.7g tetra ethoxy germanium 172.4g ethanol 839ml 0.2N hydrochloric acid 106.8g 29% aqueous ammonia 51ml Cylindrical silica glass was formed in the same way as in Example 10 using the materials shown in Table 1. All of the materials were refined by distillation, filtration, etc. 10 samples of cylindrical doped silica glass of the same size as in Example 10 (23.2mm in diameter and 417.5mm in length) were obtained with a yield of 100%.

Germanium included in the obtained doped silica glass was measured and 10 mol% of germanium was detected throughout the silica glass which showed that the yield of germanium was almost 100% and that the germanium was uniformly dispersed in the glass.

Using a rod-in-tube method, a glass made in accordance with this Example and a tube of pure silica were used to form an optical fibre of the step-index type of 125ym in outer diameter and 50Fm in core diameter. The wavelength transmission loss property of the resulting optical fibre was measured and the absorption peak due to OH groups included therein was seen when the wavelength was 1.39m and 1.410cm. For a wavelength of 1.56cm, the transmittance loss was less than 1.0dB/km and thus the obtained optical fibres proved to be of very low tranmittance loss. The OH groups included in the resulting glass was less than 100ppb in view of the absorption peak mentioned above.

As described above, by adding germanium to the hydrolysed solution, high quality doped silica glass was obtained.

Example 12 A solution composed of the same components as in Example 11 was gelled by a rotating gellation method to form a tubular wet gel. The tubular wet gel was dried and sintered in the same way as in Example 11. However, as the wet gel was tubular in this Example, whilst it was cylindrical in Example 11, the tubular wet gel could be dried more uniformly than a cylindrical gel, and the drying step was completed within 5 days. Moreover, foams and foreign particles or irregular shape which were possibly included in the glass were completely eliminated without taking great care in the process of making the tubular wet gel.

By adding germanium into the hydrolysed solution, high quality doped silica glass was obtained.

Example 13 444.6g of 0.2 normal hydrochloric acid was added to 642.5g of ethyl silicate to carry out hydrolysis and the resulting solution was referred to as a hydrolysed solution.

A mixture of 78ml of 29% aqueous ammonia, 839ml of ethanol and 271.7g of water were added to a mixture of 785.3g of ethyl silicate and 839ml of ethanol and the mixture was stirred at 30"C to yield fine particles of silica. After allowing to stand overnight, the solution was condensed under reduced pressure.

Alcohol in the condensed solution was substituted with water in order to enhance the yield during the drying step and the pH was adjusted to 4.0 by the addition of 2 normal hydrochloric acid in order to prevent gelation at the time of mixing with the hydrolysed solution. The resulting solution included fine particles of silica of mean particle diameter of 0.28im and the solution was referred to as a silica particle dispersed solution.

The hydrolysed solution and the silica particle dispersed solution were mixed and the pH of the resulting sol solution was adjusted to 4.75 by the addition of 0.2 normal aqueous ammonia and water and the volume was adjusted to 1872ml.

The sol solution was fed into a cylindrical container of polytetrafluoroethylene ((TEFLON) Trade Mark) whose inner diameter was 50mm and whose length was 1000mm, to a height of 900mm and was gelled in 40 minutes at room temperature of about 20"C.

10 wet gels formed in the same way as above were aged for 3 days and placed in a drying container having openings amounting to 0.2% of the surface area and the wet gels were dried at 60 C. After 13 days, the dry gels, which did not fracture even at room temperature, were obtained with a yield of 100%.

The bulk density of the obtained dry gels was 0.67g/cm3.

10 dry gels were placed in a sintering oven and heated to 200"C at 30"C per hour, maintained at 200"C for 5 hours, heated to 300 C at 300C per hour and maintained at 300"C for 5 hours to remove absorbed water. Then the gels were heated from 300"C to 920"C at 60"C per hour and maintained at 920"C for 2 hours to remove carbon, ammonium chloride and to accelerate the dehydrating condensation reaction.

The gels were then cooled down to room temperature to yield 10 sintered gels including a great many micro pores.

The sintered gels were dipped into a mixture of tetra ethoxy germanium and ethanol to impregnate the gels with the solution. After drying, the gels were sintered in the same way as in Example 10 and samples of cylindrical transparent glass were obtained.

The dimensions of the samples of transparent glass were 23.5mm in outer diameter and 423.6mm in length. Germanium included in the transparent glass was measured by IMA, XMA, ICP, etc. and 5 mol% of germanium was detected throughout the transparent glass indicating that germanium was uniformly dispersed.

As above, by adding germanium to the gel, high quality doped silica glass was obtained.

Example 14 444.6g of 0.02 normal hydrochloric acid were added to 642.5g of ethyl silicate to carry out hydrolysis and the resulting solution was referred to as a hydrolysed solution.

A silica dispersed solution was prepared in the same manner as in Example 10.

A mixture of tetra butoxy germanium and ethanol was hydrolysed to yield a solution in which fine particles of germanium of a mean particle diameter of less than 0.1 ism were uniformly dispersed.

The hydrolysed solution, the silica dispersed solution and the germanium particle dispersed solution were mixed at a molar ratio of silicon atom and germanium atom of 45:52:3. The preparation of the sol solution, gelation, drying and sintering were all performed in the same manner as in Example 10 and a cylindrical transparent glass was obtained. The dimensions of the transparent glass were 23.2mm in outer diameter and 200mm in length. Germanium included in the glass was measured by IMA, XMA, ICP, etc. and 3 mol% of germanium was detected throughout the transparent glass. The yield of germanium was almost 100%, including that the germanium was uniformly dispersed throughout the glass.

Thus, by adding the germanium in the form of fine particles, high quality doped silica glass was obtained.

Example 15 444.6g of 0.02 normal hydrochloric acid was added to 642.5g of ethyl silicate to carry out hydrolysis and the resulting solution was referred to as a hydrolysed solution.

Fine particles of mean particle diameter of 0.15m of solid solution of germanium of 6 mol% with respect to the silica synthesised by a gas-phase method were uniformly dispersed in water at twice molar weight to yield a fine particle dispersed solution.

The hydrolysed solution and the fine particle solution were uniformly mixed so that the molar ratio silicon to germanium in the completed glass was 97:3. Then the preparation of a sol solution, gelation, drying and sintering were all performed in substantially the same manner as in Example 10 and a cylindrical transparent glass was obtained.

The dimensions of the obtained transparent glass were 23.2mm in outer diameter and 200mm in length. The germanium included in the glass was measured by XMA, IMA, ICP, etc. and 3 mol% of germanium was detected throughout the glass, indicating that the germanium was uniformly dispersed in the glass and the yield of germanium was almost 100%.

As mentioned previously, by adding germanium in the form of fine particles, high quality doped silica glass was obtained.

Example 16 A mixture of silane tetrachloride and n-hexane was hydrolysed to yield a hydrolysed solution.

Fine particles of mean particle diameter of 0.13cm of silica and germanium at 6 mol% with respect to the silica synthesised by a gas-phase method were uniformly dispersed in a mixture of n-hexane and ethanol. The resulting solution was referred to as a fine particle dispersed solution.

The hydrolysed solution and the fine particle dispersed solution were uniformly mixed so that the molar ratio of silicon to germanium in the completed glass was 97:3. Then the preparation of a sol solution, gelation, drying and sintering were performed in the same manner as in Example 10 and a cylindrical transparent glass was obtained. The dimensions of the cylindrical transparent glass were 5mm in outer diameter and 200mm in length. Germanium included in the transparent glass was measured by XMA, IMA, ICP, etc. and 3 mol% of germanium was detected throughout the transparent glass, indicating that the yield of germanium was almost 100% and that germanium was uniformly dispersed in the glass.

In this Example, by adding germanium in the form of fine particles, high quality doped silica glass was obtained.

Example 17 Various samples of doped silica glass including different dopants were formed using different constituents. The results are shown in Tabie 2.

TABLE 2 Added to In the Form of Added to Added to Added DO- Hydrolyzed Solution Fine Particle Gel Sol Solution During Sintering Note PANT Al # #*1 - - - 30% *5 Ti # - - - - 15% *5 Zr # #*1 - - - 10% *5 Ta # - - - - 5% *5 P # - - - - 10% *5 B # #*1 - # - 15% *5 Ga # - - - - 20% *5 Li - - - # - small amount *5 Na # - - # - 20% *5 K - - - # - small amount *5 Cs - - # # - small amount *5 Tl - - - # - small amount *5 Ag - - # - small amount *5 F - #*1 - - #*3 ##0.4% *5 N - - - - x *4 fumed (Ref.) (Embo. 1,2,3) (Embo. 5,6,7) (Embo. 4) > 10% *5 Ge # #*1 # - - acceptable *2 #...A large bulk glass was obtained #...A bulk glass was obtained x...A bulk glass was not obtained -...Not examined *1...The dopant was added to fine particle silica.

*2...The mixture of fine particles of dopant and silica *3...SF6 gas *4...N2 gas *5...The amount of doping In this Example, by adding the various dopants by various method, different samples of doped silica glass were obtained.

It will be appreciated that a method according to the present invention of preparing doped silica glass enables pure cylindrical doped silica glass of high quality to be produced from which optical fibres can be manufactured. In particular it enables cylindrical doped silica glass, for example, of 2cm in outer diameter and 1m in length to be produced which was impossible heretofore.

In summary, the method of the present invention and described above provides high quality cylindrical doped silica glass of large size and good yield and by drawing the doped silica glass and coating it with a low refractive index material such as plastics, optical fibres may be manufactured at low cost. Moreover, coloured silica glass can be obtained also at low cost. Moreover, the refractive index of the silica glass may be adjusted as necessary by suitably selecting the dopant. Silica glass made by a method according to the present invention is not only applicable in the manufacture of optical fibres but also in the manufacture of material for crad, optical glass, construction material, etc.

Claims

1. A method of preparing doped silica glass comprising the steps of: preparing a sol solution by mixing a hydrolysed solution and a solution including ultra fine particles of silica; gelling the sol solution to produce a gel; drying the gel to produce a dry gel; sintering the dry gel to produce silica glass; and adding a dopant to one or more of the hydrolysed solution, the solution of ultra-fine particles of silica, the sol solution, the gel or during drying or sintering.

2. A method as claimed in claim 1 in which the hydrolysed solution comprises a solution of an alkyl silicate.

3. A method as claimed in claim 1 or 2 in which the solution of ultra fine particles of silica is prepared by hydrolysing an alkyl silicate with aqueous ammonia or ammonia gas and water.

4. A method as claimed in claim 1 or 2 in which the solution of ultra fine particles of silica is prepared by dispersing ultra fine particles of silica in water.

5. A method as claimed in any preceding claim including adjusting the pH of the sol solution to a predetermined value by adding ammonia, aqueous ammonia or an organic base.

6. A method as claimed in claim 5 in which the organic base is one or more of triethylamine, pyridine or aniline.

7. A method as claimed in any preceding claim in which the dopant is a metal alkoxide.

8. A method as claimed in claim 7 in which the metal alkoxide is tetra alkoxy germanium.

9. A method as claimed in claim 2 or any of claims 5 to 8 when dependent thereon in which the hydrolysed solution is prepared by partially hydrolysing the alkyl silicate with water in a molar ratio between 1 and 3 with respect to the alkyl silicate at a temperature below 20 C, adding the dopant and completing hydrolysation.

10. A method as claimed in claim 2 or any of claims 5 to 8 when dependent thereon in which the hydrolysed solution is prepared by adding water to a solution of the alkyl silicate and alcohol at a volume ratio of more than 20% with respect to the alkyl silicate, said water being in a molar ratio of between 1 and 3 with respect to the alkyl silicate, to carry out partial hydrolysation, adding the dopant, and completing hydrolysation.

11. A method as claimed in any preceding claim in which the mean particle diameter of the ultra fine particles of silica is between 0.01m and 1.0cm.

12. A method as claimed in claim 4 in which the ultra fine particles of silica are dispersed by ultrasonic vibration andlor centrifugal separation.

13. A method as claimed in any preceding claim in which the temperature and pH of the sol solution is adjusted so that gelation of the sol solution is completed in between 3 minutes and 100 minutes.

14. A method as claimed in any preceding claim in which the composition of the sol solution is such that the volume of the silica glass obtained by drying and sintering the wet gel is between 5 and 15% of the volume of the wet gel.

15. A method as claimed in any preceding claim in which the wet gel is dried in a cylindrical container having lids at each end, each lid having openings amounting to less than 15% of the surface area thereof.

16. A method as claimed in any of claims 1 to 14 in which the wet gel is removed from a cylindrical container and is dried in a further container having openings amounting to less than 15% of the surface area thereof.

17. A method as claimed in any preceding claim in which the wet gel is dried at a temperature between 0 C and 100"C and heating to a temperature between 20"C and 1200C at a rate of less than 1200C per hour.

18. A method as claimed in any preceding claim in which the dry gel is sintered by removing absorbed water, removing carbon, removing hydroxide groups, removing chloride and/or fluoride ions, and making the dry gel non-porous.

19. A method as claimed in claim 18 in which the absorbed water is removed by heating the dry gel to a selected temperature between 20"C and 400"C at a rate less than 400"C per hour and maintaining the dry gel at the selected temperature for at least one hour.

20. A method as claimed in claim 18 or 19 in which carbon is removed by heating the dry gel to a selected temperature between 400"C and 1200"C at a rate between 30"C and 400"C per hour and maintaining the dry gel at the selected temperature for at least one hour.

21. A method as claimed in any of claims 18 to 20 in which the hydroxide groups are removed by heating the dry gel to between 700 C and 1200"C in a stream of a carrier gas of helium, neon, argon, nitrogen or oxygen or mixtures thereof and a hydroxide group removing agent in a ratio between 1 and 40% with respect to the carrier gas, in a sintering oven.

22. A method as claimed in claim 21 in which the hydroxide group removing agent is Cl2, SOCI, SF6, CF4, C2F6 or C3F.

23. A method as claimed in any of claims 18 to 22 in which, after removing the hydroxide groups, chloride and/or fluoride ions are removed by heating the dry gel to between 700"C and 1200"C in an oven in a stream of a carrier gas of helium, neon, argon or nitrogen or mixtures thereof.

24. A method as claimed in any of claims 18 to 23 in which the dry gel is made non-porous by heating the dry gel to between 1000"C and 1500"C in a stream of helium gas.

25. A method as claimed in any of claims 18 to 24 including the step of, after making the dry gel nonporous, the dry gel is heated to between 1000"C and 1600 C and maintained at a selected temperature for a given period of time to produce a transparent silica glass.

26. A method as claimed in any of claims 18 to 25 in which the heating rate during the removal of hydroxide groups, chloride and/or fluoride ions and making the dry gel non-porous, is between 0.5 and 1000"C per minute.

27. A method substantially as herein described with reference to Examples 1 to 17.

28. Doped silica glass when prepared by a method as claimed in any preceding claim.

29. A method of preparing doped silica glass comprising the steps of: preparing a sol solution by adding ultra fine particles of silica into the hydrolysed solution, gelling said solution to a gel, drying said gel to a dry gel, sintering said dry gel to a glass, and at least one step for adding dopants selected from the group consisting of: (i) adding dopant into said hydrolysed solution, (ii) adding dopant in the form of fine particle, (iii) adding dopant into said sol solution, (iv) adding dopant into said gel, and (v) adding dopant during sintering.

30. A method of preparing cylindrical doped silica glass comprising the steps of: preparing a sol solution by mixing the hydrolysed solution in which a suitable metal alkoxide (M(OR)x wherein M is a metal) is added to alkyl silicate (represented by the general formula Si(OR)4 wherein R stands for an alkyl group) at a molar ratio of 0% or more and the ultra fine particle of silica yielded by hydrolysing alkyl silicate with ammonia water or ammonia gas and water, said metal alkoxide being included as an agent for adjusting the refractive index (hereinafter referred to as "dopant"), adjusting the pH of said sol solution to a predetermined value by the addition of ammonia, ammonia gas, an aqueous solution of ammonia or organic base (particularly triethylamine, an aqueous solution of triethylamine, pyridine, an aqueous solution of pyridine, aniline or an aqueous solution of aniline), casting said sol solution to a hydrophobic cylindrical container, gelling said sol to a wet gel, drying said wet gel to a dry gel, and sintering said dry gel to a transparent glass.

31. Any novel integer or step or combination of integers or steps, hereinbefore described, irrespective of whether the present claim is within the scope of, or relates to the same or a different invention from that of, the preceding claims.