GB2181727A - Method of preparing a silica glass member - Google Patents

Abstract

A method of preparing a silica glass member comprising the steps of: drying a sol solution containing a silica compound so as to form a dry gel, obtaining a glass or a glass precursor member by closing pores in said drygel, and obtaining a silica glass member by heating said glass or glass precursor member to a selected temperature substantially in the range between 1500 and 2200 DEG C and maintaining the said glass or glass precursor at said temperature for a predetermined period of time.

Description

SPECIFICATION Method of preparing a silica glass member This invention relates two a method of preparing a silica glass member.

Silica glass is used for many purposes such as glass wafers, semiconductors industrial material, optical material, preformsforoptical fibres, a support tube and a photo mask substrate, andthusthe demand for silica glass is expected to expand more and more in the future.

So far as the method of preparing silica glass at a low cost is concerned, the sol-gel method has been proposed. For example, this method is disclosed in "Journal of Non-Crystalline Solids" Vol.37, No.191 (1980) by Nogami etal, "Journal of Non-Crystalline Solids" Vol, 47, No.435 (1982) by Rabinovitch etal, U.S.

PatentApplication Serial No.642,606 byToki et al (corresponding to British PatentApplication No.84/18301), and U.S. PatentApplication Serial No.826,527 by Matsuo et al (corresponding to British PatentApplication No.86/03421).

The difference between the various methods referred to in the above literature concern the components of the sol solution which is to be the main material, and the methods can be classified into thefollowing four groups: 1) The use of a sol solution obtained by hydrolyzing a mixture solution of alkyl silicate, water, alcohol and an appropriate catalyst such as hydrochloric acid, ammonia and soon (Nogami's method), 2) The use of a mixture sol solution obtained by mixing a solution oaf alkyl silicate hydrolyzed with an acid reagent and a solution including fine particle silica obtained by hydrolizing alkyl silica with a basic reagent ata predetermined mixing ratio (Matsuo's method).

3) The use of a mixture sol solution obtained by mixing a solution ofalkyl silicate hyrdolyzed with an acid reagent and ultra fine particle silica at a predetermined mixing ratio (Toki's method), and 4) The use of a sol solution obtained by diffusing ultra fine particle silica into water organ organic solvent at a predetermined ratio (Ravinovitch's method).

The resulting sol solution prepared by each method as mentioned above is gelled in a container ofthe desired shape and the resulting dry gel is sintered to yield silica glass. Each of the methods as mentioned above has merits and demerits of its own, and the features of each method are shown in the following table.

Nogami Matsuo Toki Rabinovitch Purityofthesilicaglass 8 Q A A Cost of raw material 0 A 0 O Yield X Q Q O Size X O O A Mass-productivity X A O 0 The symbols employed in the above Table have the following meanings: O excellent O good A satisfactory.

X ................ unsatisfactory So far as mass production is concerned, Toki's method is the best ofthe above four methods, and so far as the physical properties of the product in terms of high purity are concerned, Matsuo's method is the best ofthe above four methods.

However, when a sol solution as described above is used as the starting material and is only dried and sintered,there are many inclusions in the silica glass member so obtained. In orderto enhance the quality of the glass, Matsuo et al have succeeded in removing any inclusions which are largerthan afew microns, by using a clean environment, by irradiating the sol solution with supersonic waves in order to improve the dispersion properties by filtering, and by centrifugal separation. Matsuo et al have also succeeded in prevent ing bubbling by closing the pores in the dry gel as a result ofsintering in an He atmosphere or under reduced pressure. The inclusions in the silica glass have been substantially reduced by this method.However, it has not so far been possible to produce silica glass which does not include silica crystals, inclusions, micro-cracks, bubbles and soon. The silica glass which is presentlyavailablestill cannot be used in anyfield whereveryhigh quality is required,such as in the production of a photo mask substrate and a preform for an optical fibre.

According, therefore, to the present invention, there is provided a method of preparing a silica glass member, comprising the steps of: drying a sol solution containing a silica compound so as to form a dry gel, obtaining a glass or a glass precursor member by closing pores in said dry gel, and obtaining a silica glass member by heating said glass or glass precursor memberto a selected temperature substantially in the range between 1500 and 2200'C and maintainingthesaid glass or glass precursoratsaid temperature for a predetermined period oftime.

The invention enables a silica glass memberofoptically high qualityto be produced which can be used asa photo mask substrate or as a preform for an optical fibre. In addition, the present invention enables the mass-production of silica glass and also the moulding of silica glass to be improved.

The sol solution may be obtained by hydrolyzing alkyl silicate with water and with an acid or basic reagent.

Alternatively, the sol solution may be obtained by mixing together at a predetermined mixing ratio a solution of alkyl silicate hydrolyzed with an acid reagent and a solution including particulate silica obtained by hydrolyzing alkyl silicate with a basic reagent.

Afurther possibility is that the sol solution is obtained by mixing together at a predetermined mixing ratio a solution of alkyl silicate hydrolyzed with an acid reagent and particulate silica.

Furthermore, the sol solution may be obtained by diffusing particulate silica into water or into an organic solvent at a predetermined ratio.

Preferably the particulate silica has a mean particle diameter between 0.01 and 1.0 microns.

The pores in the said dry gel may be closed by sintering said dry gel in an He atmosphere, sintering the dry gel under a reduced pressure, or sintering the dry gel under a reduced pressure after the dry gel has been processed in an He atmosphere.

A gas burner may be used for heating said glass or glass precursor member to the said selectedtemperature.

The glass or glass precursor member may be heated to the said selected temperature by means of a high temperature furnace having a graphite, a tungsten or a molybdenum heating element.

Alternatively, the glass or glass precursor member may be heated to the said selected temperature by means of a high temperature continuous heat-treating furnace. The glass or glass precursor member may be heated to the said selected temperature by means of a high temperature gas furnace in which the combustion hydrogen our a hydrocarbon gas is used as a heat source.

Spacer means may be provided between the glass or glass precursor member and a supporttherefor.

The said spacer means may be constituted by carbon powder, carbon fibres or paper-like orfabric-like material obtained by processing carbon fibres.

The said spacer means may alternatively be constituted by a powder which is hard to sinter. Thus the said powder may be alumina, zirconia, or silicon nitride.

A number of said glass or glass precursor members may be simultaneously heat-treated while mounted on said spacer means.

The said glass or glass precursor member or members may be moulded by using a casting of desired configuration, when said glass or glass precursor member is heated to the said selected temperature.

Alternatively, the glass or glass precursor member or members may be moulded to a desired configuration by subjecting the said member to an external force when said glass or glass precursor member is heated to the said selected temperature.

Preferably, afterthe glass or glass precursor member has been heated to the said selected temperature, it is cooled to room temperature in a plurality of stages, the cooling rate being smaller in at least one of said stages than in another stage or stages thereof.

One of the merits ofthe sol-gel method is that a high melting point glass can be synthesized at a low temperature. In fact, if the silica glass is manufactured by the fusing method, a very difficult manufacturing step at a temperature higher than 1700"C is necessary. On the other hand, ifthe silica glass is manufactured by the sol-gel method, it is easy to manufacture the glass at a low temperature of about 1200"C.

The temperature of vitrification depends upon the component ofthe sol solution which is the starting material and, for example, when alkyl silicate is hydrolyzed with an acid reagent or with a basic reagent, the temperature of vitrification is 900"C and 1200 C, respectively.When the solution obtained by hydrolyzing alkyl silicate with the acid reagent is mixed with fine particle silica obtained by hydrolyzing alkyl silicate with the basic reagent or with ultra fine particle silica, although the temperature of vitrification also depends upon the mixing ratio thereof, the vitrification is completed at a temperature lowerthan 1400"C. The temperature of vitrification is highestwhen a sol solution is used in which ultra fine particle silica is diffused into the solvent, but even in this case the vitrification is completed at a temperature less than 1470"C.

The use of the sol-gel method, therefore, enables the silica glass to be manufactured with less energy in comparison with that required in the fusing method.

However, there are some inclusions, defects and soon in the silica glass manufactured by the sol-gel method whatever may be the components of the sol solution. These defects arisefrom:- 1) Inorganic matter, for example, dust mixed in the raw material and the sol solution, 2) Defects produced by the burning out of the organic inclusions, 3) Micro-cracks occurring at the time of shrinking, 4) Bubbles or blow holes included at the time ofgelation or produced during the sintering step, 5) Silica crystals produced during the sintering step (mainly, crystalite), and 6) Siiica-coagulated material which is insufficiently sintered.

In the case of the present invention the glass or glass precursor member which may be obtained bythe conventional sol-gel method is heated to a temperature close to the melting point of silica and thus the glass or glass precursor is broughttemporarily to the half-fusing phase. The crystals mentioned in (5) above and the silica-coagulated material mentioned in (6) above disappear at a temperature higher than that of silica, and even if the temperature is lowerthan the fusing temperature, an almost uniform silica glass is produced.

Moreover, the defects of (2), the micro cracks of (3) and the bubbles of (4) and soon disappear during the process ofsintering by heating the dry gel to the high temperature if the pores in the dry gel are closed in an He atmosphere or under reduced pressure.

The inorganic matter of ), even if the melting point is higherthatthatofsilica, disappearopticallyduring the process of sintering by heating the dry gel to a temperature close to the melting point of silica, because the interface of (1) and the silica glass disappears and the composition ofthe glass is made uniform. However, when extremely large particles, or inorganic materials which are hard to vitrify, are included, the quality ofthe glass is not made completely uniform. It is desirable to remove particles whose size is largerthan afew microns by performing the sol processing in a clean environment and also by filtering or centrifugal separating.

Since the melting point of silica is 1713"C, if this temperature orone higher is maintained in themanufacturing process, the high quality of the silica glass is assured. So far as improving the quality is concerned, there is an adequate improvement at a temperature of 1500"C or higher. Accordingly, the high temperature treatment employed in the present invention starts at a temperature of at least 1 500"C in view of the quality required,the consumption of energy and the characteristics of the furnace. However, since too high a temperature causes violent vaporization of the silica, the upper limit of the temperature is substantially 2200"C.

As mentioned above, the present invention is applicable regardless ofthe components ofthe sol solution.

However, unless the pores in the dry gel are closed by one of the following methods, the pores grow into very large bubbles: 1) sintering and closing pores in the dry gel in an He atmosphere, 2) sintering and closing pores in the dry gel under reduced pressure, and 3) sintering and closing pores in the dry gel under reduced pressure after the dry gel has been processed in an He atmosphere.

In the step of closing pores in the dry gel, it is not always necessary to completely virtifythe dry gel butthis can be done if a translucent glass precursor is obtained. There may be several methodsforheating a glass or glass precursor member, after closing the pores to a selected temperature between 1500 and 2200"C.

First, use may be made of a gas burner of hydrogen, acetylene and soon. The gas burner is easily available and the operation thereof is easy. However, it has some disadvantages in that the control of the temperature is very difficult and the difference between the temperature of the surface and that of the inside of the glass is considerable. Moreover, this method is not suitablefor mass-production.

Second, use may be made ofa high temperature furnace in which graphite, tungsten, molybdenum orthe like is used as the heating element. This, however, is very expensive and its operation is difficult in that, for example, this furnace is used in the atmosphere where there is no oxygen. However, an accurate temperature control is assured and high quality silica glass can be manufactured with a stable yield. Furthermore, the high temperature furnace can also be used as a high temperature continuous heat-treating furnace by combining the heating elements, which is useful for mass production.

Besides the above, an high temperature gas oven may be used in which the combustion of hydrogen or hydrocarbon gas is used as the heat source.

In the production of a photo mask substrate, a wide square of, for example, 5 inch (12.7 cms) x 5 inch (12.7 cms) x 0.09 inch (0.23 cms) or6 inch (1 5.24cms) x 6 inch (1 5.24cms) x 0.12 inch (0.30 cms) is required in addition to stable quality. To satisfy both of these requirements, a high temperature furnace may be employed in which graphite,tungsten or molybdenum is used as the heating element. However, since the samples are treated in the half fused state, the samples are liable to be welded to thefurnace material, thereby causing, cracks and transformation. Therefore, it is desirable to provide an isolating layer between the furnace material and the samples to avoid the cracks and transformation.

Since carbon material is chemically very stable in an inert atmosphere, it does not react on the silica glass, thereby making it easy to obtain a material of high purity. If an isolating layer of powder or fibres is used,the difference between the expansion coefficients of the samples and ofthefurnace material is absorbed bythe displacement of the isolating layer, thereby preventing the cracks and the transformation. Moreover, even though a part of the isolating layer of carbon powderorcarbon fibres adherestothe sample, it is easyto remove it by washing or burning. lfcarbon processed to a paper-like orfabric-likeshape is used astheisolating layer, the handling becomes much easier and the state ofthe contact surface becomes better.If an isolating layer is provided between one sample and another, the same effect as mentioned above is obtained, and space in the furnace is substantially improved, so that mass production is made easier.

If an high temperature furnace is employed in which tungsten or molybdenum orthe like is used asthe heating element, then if there is carbon present, the heating element tends to become carbonized and accordingly deterioriates. Therefore, in such a case, powder which is hard to sinter, such as alumina, zirconia, silicon nitride orthe like could be used as the isolating layer. However,any powderwhich adherestothe sample is hard to remove.

If the glass or glass precursor member is heated to the selected temperature between 1500 and 2200 C,the sample becomes soft and easilytransforms. One ofthe merits ofthe sol-gel method is the ease with which the product maybe moulded atthe time of gelation. Furthermore, it is also possible to mould the gel during the high temperature treatment. For example, when a large square silica glass plate isto be manufactured, its flat shape is not always maintained during vitrification even if the gelation is carried out in a container such as a flat plate casting. When the sample is placed on a furnace plate having aflatsurface and is heated to the high temperature, the sample is flattened by its own weight. This makes it suitable for subsequent grinding.

If a casting of a desired configuration, whether flat or not, is used, an extremely precise moulding can be produced. It is also possible to apply pressure to the samples by a press device provided in the furnace instead of depending upon the weight of the sample itself.

If a silica glass rod oratube isto be manufactured, itwould be more efficientto use a ring burner or a ring heater. When the central portion ofthe rod ortube is heated to the high temperature with both ends ofthe rod or tube fixed, by subjecting both ends to tension, the straightness of the rod ortube is improved. The method mentioned above is very important when the invention is used in connection with the production ofthe preform and the supporttube of an optical fibre.

If the glass or glass precursor member is heated to the selected temperature between 1500 and 2200 C and cooled down rapidly, the internal stress remains in the silica glass. Therefore, eitherthe cooling down afterthe high temperature treatment may be gradual orthere may be annealing after rapid cooling.Thus, inthe manufacturing process of the glass, at least one step of cooling down the material graduallyfrom 1 200"to room temperature should be included.

The high temperature treatment according to this invention is basically different from the fusing method.

The major differences between the present invention and the fusing method are asfollows:-the bulk ofthe silica glass itself has been already moulded by the sol-gel method ofthe prior art, the length of the high temperature treatment being even less than that of the fusing method and almost no work is required at the time of high temperature treatment. A heat-treatment is similar two an annealing treatment to remove distortion in glass, and therefore, this is regarded as a treatment for removing inclusions in the glass. As explained above, according to the present invention, an improved silica glass member of high quality and high mouidability, which cannot be manufactured by2he prior artsol-gel method, can be produced.Further,the present invention provides such a silica glass at the lower costthan in the prior art.

Furthermore, a multicomponent series glass such as, for example, an alkali-proof glass of the SiO2-ZrO2 series or a low thermal expansion coefficient glass oftheSiO2-TiO2series may also be provided with high quality and at low cost by the use ofthe present invention.

As previously indicated, an optically high quality silica glass which can be used as a photo mask substrate and as a preform foran optical fibre can be supplied ata low cost in large quantities.

The invention is illustrated by the following Examples.

Example 1 440ml of ethyl silicate and 360ml of 0.05 normal hydrochloric acid solution were mixed together and stirred violently to yield a water-while transparent uniform solution. The pH of the solution so obtained was adjusted to 4.2 with 0.1 normal ammonia water and then the solution was filtered through a filter having holes whose diameter was 1 micron. 500ml of the resulting solution was poured into a container made of polypropylene (whose dimensions were 20 cms in width, 20 cms in length and 10 cms in height). The solution was gelled and dried forten days at a temperature of 60 C in the said container which was covered with a lid with openings amounting to 0.5% of the surface area of the lid, thereby obtaining a water-whitetransparent dry gel.

The dry gel was placed in a gas displacement furnace and the temperature thereof was raised to 7000C at a heating rate of 30"C per hour. When the temperature became 700"C, pure helium gas was introduced in an amount of 1cumin into the furnace, the temperature was raised to 9000C at a heating rate of 1 00C per hour and the dry gel was maintained at900 Cforone hour. As a result, the dry gel was made into a clearglassmember whose specific gravity was 2.20. The size thereof was 8cm x 8cm x 0.5cm. Only a few inclusions in the member having a diameter of a few microns were detected.

By use of a gas burner, an oxy-hydrogen flame was applied to both sides ofthe silica glass memberso obtained. When the temperature ofthe surface of the silica glass member was at least 1800C,the silica glass memberwas maintained for morethan ten seconds atthistemperature and thewhole surfacethereofwas heated to an almost uniform condition. As a result, no inclusions were detected by a microscope having a magnification of 100, although the whole surface exhibited distortions. After the silica glass member had been maintained at 1 200'C for one hour, thetemperature was lowered at a cooling rate of 1 OO"C per hour in orderto removethe distortions.The silica glass member so obtained was mirror-polished into a glass memberof2mm in thickness and then collimated from a lamp was directed onto itso thatthe illumination was 50,000 lux in a dark room. However, there were no reflectance points.

Example 2 440my of ethyl silicate, 900ml of ethanol and 360my of 0.1 normal ammonia water were mixed uniformly and were maintained for one day at room temperature. The resulting emulsion sol solution was concentrated to 440my in total volume by using a rotary evaporator. The said sol solutionwasfiltered by meansofafilter having holes whose diameter was 1 micron, and 440ml ofthe resulting solution was poured into a container (of 5cm inside diameter, and 30cm depth) made of polypropylene having a lid with openings amounting to 2% of the surface area ofthe lid. Afterthe solution had been dried at 60"forten days, a white dry gel was obtained.

The dry gel so obtained was placed in a vacuum furnace and the temperaturewas raised to 900"C at a heating rate of 60"C per hour. The atmospheric pressure in the vacuum furnace was lowered to less than 1 Torr by using a rotary pump and then,while maintaining the pressure, thetemperature was raised to 1200 at a heating rate of 100 C per hour. After the dry gel was maintained at the temperature of 1 200"C for one hour, a vitrification of the dry gel occurred. The specific gravity thereof was 2.20 and a silica glass rod was obtained, the diameter and length thereof being 2.5cm and 10cm, respectively.When the silica glass rod was exposed to the irradiation of laser light whose wavelength was 0,633m, the light was scattered everywhere.

The rod was fixed on a glass lathe and was heated by an oxy-hydrogenflamewhile rotating. When the surface temperature was at least 20000C, the rod was maintained at this temperature for at least 30 seconds and then the rod was heated uniformly by sliding the burner along it. When laser light was directed onto the silica glass rod again, no scattering ofthe lightwas observed.

Example 3 440ml ofthe ethyl silicate, 900ml ofthe ethanol and 360ml of 0.1 normal ammonia water were mixed uniformly and were maintained for one day at room temperature. Afterthe resulting emulsion sol solution had been concentrated to 440my in total volume by using a rotary evaporator, the pH thereof was adjusted to 4.0 by adding 1.0 normal hydrochloric acid solution.

Separately from the above, a water-white transparent uniform solution was obtained by violently stirring a mixture of 440ml of ethyl silicate and 360ml of 0.05 normal hydrochloric acid solution. This solution was uniformly mixed with the sol solution obtained above and was then filtered by means of a filter having holes whose diameter was 1 Fm. The pH ofthe solution was adjusted to 4.8 with 0.1 normal ammoniawaterandthen 1,000my ofthe resulting solution was poured into a container made of TEFLON (Registered Trade Mark), the container being 6cms inner diameter and 40cm length and being sealed with a stopper.After setting the sol in the container on a revolving device and rotating the container about the central axis of the said device at a rotational speed of 500rpm for one hour, the container was left at rest for two days.

After taking offthe stopper of the container, the dry gel was removed from the latter and placed in another container (10cm width, 45cm length, 15cm height) made of polypropylene which was then covered with a lid, the latter having openings amounting to 1%ofthe surface area ofthe lid. When the dry gel was dried at 60"four ten days, a tubular dry gel was obtained.

Thetubulardrygel was placed in a vacuum furnace and was heated to 800'C ata heating rate of 600C per hour. Afterthe atmospheric pressure inthevacuum furnace had been lowered to 1 Torror less atthe temperature of 800"C, pure helium gas was passed over the gel at 1 t/min in thefurnace.

The atmospheric pressure in the vacuum furnace was lowered to 1 Torr or less again and then, while maintaining this pressure, the temperature of the vacuum furnace was raised to 1 200"C at a heating rate of 1000C per hour. Afterthe dry gel had been maintained at the temperature of 1200"Cfor one hour, the drygel was vitrified. The specific gravity thereof was 2.20 and the product was in the form of a silica glass tube whose outer diameter, inner diameter and length were 3cm, cm and 20cm, respectively. When this silica glass tu be was exposed to irradiation by laser light whose wavelength was 0.633ELm, the light was scattered everywhere.

The silica glass tube was placed in a graphite heating furnace vertically and afterthe introduction of N2 gas in substitution for the original atmosphere, the temperature therein was raised to 1600"C in two hours and was maintained atthis temperatureforten minutes. Thetemperature wasthen lowered to 1200"C atthe rateof 1000C per hour and was then further lowered to room temperature at the rate of 100 C per hour. When the silica glass tube was exposed to irradiation by laser light, very little scattering ofthe light was observed.

Example 4 440my of ethyl silicate and 360ml of 0.05 normal hydrochloric acid solution was mixed together and stirred violently to obtain a water-white transparent uniform solution. 1509 of ultra fine particle silica (Aerosil OX-50) was added little by little to the water-white transparent uniform solution and the solution was stirred sufficiently. This sol solution was exposed to irradiation by ultrasonicwaves of 28KHz for two hours ata temperature of 20"C, and after a centrifugal force of 1 500G was applied for ten minutes to remove large particle silica, the so solution was filtered by means of a filter having holes whose diameter was 1 micron.

The pH ofthe highly homogeneous sol solution obtained above was adjusted to 4.2 with 0.1 normal ammonia water. 500ml of the resulting solution was poured into a container (20 cm width x 20cm length x 10cm height) made of polypropylene having a lid whose openingsamountedto 1% ofthesurface area ofthe lid. When this sol solution was dried at atemperature of 60"C for seven days, a white and porous dry gel was obtained.

The dry gel was placed in a gas displacement furnace and heated to 10000C at the rate of 60 per hour. When the temperature became 1 000 C, pure helium gas at a flow rate of 1e/min was introduced into thefurnace and the dry gel was heated to 1300"C at the rate of 30"C per hour. When the dry gel was maintained at 13000 forone hour, the vitrification of the dry gel was completed and a silica glass plate was obtained whose specific gravity was 2.20.The size ofthe silica glass plate was 1 Ocm x 1 Ocm x 0.5cm. inclusions and ss crystalite crystals whose diameterwas about 10 micron were detected to a small extent in the silica glass plate.

Carbon powderwas placed on a graphite plate (15cm x 15cm x 1cm)to a depth of about 1 mm.Thesilica glass plate was mounted to the carbon-covered graphite plate and this assembly was placed in a graphite heating furnace. After the furnace atmosphere had been replaced by N2 gas, the temperaturetherein was raised to 1 8000C in two hours and was maintained at this temperature for ten minutes. The temperature was then lowered to 1 200 C at a cooling rate of 1 0000C per hour and was the further lowered to room temperature at acooling rateof 100 perhour.

There was no fusion between the graphite plate and the silica glass plate and the flatness of the silica glass was excellent. The silica glass plate so obtained was mirror-polished into a plate of 2mm thickness and then the collimated light from a lamp was directed onto it so that the intensity of the illumination was 50,000 lux ion a dark room. However, no reflectance points were detected. Optically, a very high quality silica glass without crystals or distortion was obtained.

Example 5 250g of ultra fine particle silica (Aerosil 200) was scattered into 500ml of pure water and this sol solution or scurry was exposed to irradiation by ultrasonic waves of 28KHzfortwo hours at a temperature of 20"C.400ml ofthe slurry which had a high viscosity, was poured into a container (5cm inner diameter, 30cm depth) made ofpolypropyleme and having a lid with openings amounting to 2% of the surface area ofthe lid.Whenthis slurrywas dried at60 Cforten days, a white porous dry gel was obtained.

This dry gel was placed in a gas displacement furnace and heated to 1 1 OO"C at the rate of 60"C per hour.

Whenthetemperature became 1 100C, pure helium gas was introduced into the furnace at a flow rate of 1Umi n, and the dry gel was heated to 1400"C atthe rate of 1400 Cforone hour, althoughthesilica glass precursor rod which was so obtained was translucent, the specific gravity thereof was 2.20.

The silica glass precursor rod was placed in a high-temperature gas furnace vertically, and heated to 1 800"C by a propane gas flame and was maintained atthis temperature forten minutes. The temperature wasthen lowered to 12000 at the rate of 1 0000C per hour and was then further lowered to room temperature at the rate of1000 per hour.

As a result a transparent silica glass rod was obtained without blowholes. The diameter and length ofthe transpa rent silica glass rod were 4cm and 24cm, respectively. When the transparentsilica glass rod was exposed to the irradiation of laser light whose wavelength was 0.633m, no scattering of the light was observed.

Example 6 Zirconia powders were laid on a tungsten plate (15cm x 15cm X 0.2cm) so that the thicknessthereofwas about 1 mm.Asilica glass plate obtained by closing the pores in the dry gel bythe same method as in Example 4was placed thereon and the assembly was introduced into a tungsten heating furnace. Afterthefurnace atmosphere had been replaced by N2 gas, the temperature therein was raised to 1 8000C in two hours and maintained at this temperature forten minutes. The temperature was then lowered to 1 2000C at a cooling rate of 1 000"C per hour, and was then further lowered to room temperature at a cooling rate of 1 OO"C per hour.

The tungsten plate was not fused with the silica glass plate and the flatness ofthe silica glass plate was excellent The silica glass plate obtained above was mirror-polished to provide a plate of 2mm thickness and then collimated lightfrom a lamp ways directed onto it so thatthe intensity of illumination was 50,000 in a dark room. However, no reflectance points were detected. Optically, a very high-quality silica glass member without crystals or distortion was obtained.

Example 7 1760 ml of ethyl silicate, 2690ml of ethanol and 670mi of 1.0 normal ammonia waterwere mixed uniformly and were maintained for five days at room temperature. After400ml of pure water had been added to the resulting emulsion, the sol solution was concentrated to 1000ml in total volume by using a rotary evaporator.

The pH of the sol solution was also adjusted to 4.0 by adding 2.0 normal hydrochloric acid solution.

Separately from the above, a water-white transparent uniform solution was obtained by stirring violently a mixture of760ml of ethyl silicate and 250ml of 0.02 normal hydrochloric acid solution. The resulting solution was uniformly mixed with the sol solution obtained above, and then filtered by a filter having holes whose diameter was 1 micron. After the pH ofthe solution had been adjusted to 4.2 with 0.1 normal ammonia water, a centrifugal force of 1500 G was applied forten minutes to remove large particle silica.

The resulting solution was then filtered again by means of a filter having holes whose diameter was 1 micron.

1100my of the resulting sol solution having a high degree of homogenitywas poured into a container (30cm x 30cm x 15cm) height made of polypropylene, the container having a lid with openings amounting to 0.5% of the surface area of the lid. When this sol solution was dried at a temperature of 60"C fortwenty days, a white and porous dry gel was obtained (22cm x 22cm x 0.9cm).

The resulting dry gel was placed in a gas displacementfurnaceand dryairwas caused to flow over it ata flow rate of 2t/min. The dry gel was heated to 7000Catthe rate of 60"C perhourand maintained at700 Cfor twenty hours. Helium gas, instead of the dry air, was introduced into the furnace at a flow rate of Pe/min, and then the dry gel was maintained at 9000C, 11 000C and 1 200"C for ten hours respectively. Vitrification of the dry gel was thus completed and the size and the flatness thereof were such as to yield a silica glass plate of 15.5cm x 15.5cm x 0.6cm, and 2mm respectively.

Carbon paper, (e.g. KUREHA carbon fibre paper) as thick as 0.3mm was laid on a graphite plate (20cm x 20cm x 1cm). The silica glass plate was mounted thereon and the assemblywas placed in a graphite heating furnace. After the furnace atmosphere had been replaced by N2 gas, the temperature therein was raised to 1850"C in two hours and maintained at this temperature forfive minutes. The temperature was then lowered to 1 200"C at a cooling rate of 1 000"C per hour and further, lowered to room temperature at a cooling rate of 100 C per hour.

The graphite plate was not fused with the silica glass plate and the flatness of the silica glass plate was accurate to 0.1 mm or less. The silica glass plate which was so obtained was mirror polished into a plate of 6 inch (15.24cms) x 6 inch (15.24cms) x 0.12 inch and 0.3 cms and then collimated lightfrom a lampwas directed onto it so that the intensity of illumination was 50,000 lux in a dark room. However, no reflectance points were detected. When the transmittance of ultraviolet lightthrough the silica glass plate was measured, for a wavelength of 200nm or more, the transmittance was constantly 90% or more and no specific absorption was observed.

Example 8 11 some of ethyl silicate and 620ml of 0.01 normal hydrochloric acid solution were mixed together and stirred violently to obtain a water-white transparent uniform solution. 300g of ultra fine particle silica (Re- olosil QS-102) were added little by little to the above solution with stirring and this sol solution was exposed to irradiation by ultrasonic waves of 28KHzfortwo hours at a temperature of 20"C. After a centrifugal force of 1 500G had been applied forten minutes to remove the large particle silica, the sol solution was filtered by means of a filter having holes whose diameter was 1 micron.After the pH of the resulting solution was adjusted to 4.2 with 0.1 normal ammonia water, the centrifugal force of 1500G was applied again forten minutes and the sol solution was filtered by means of after having holes whose diameter was 1 micron.

11 00m1 of the resulting sol solution having a high degree of homogenity was poured into a container (30cm width and 30cm length x 15cm height) made of polypropylene having a lid provided with openings amounting to 0.5% of the surface area of the lid. When this sol solution was dried at a temperature of 60"C for 20 days, a white and porous dry gel was obtained The dry gel was placed in a gas displacementfurnace and dry air was introduced into the furnace ataflow rate of 2t/min. In the course of being heated to 700"C at the rate of 60"C per hour, the dry gel was maintained at 200, 300 and 500 for three hours, respectively.Helium gas was then introduced instead of the dry air ata flow rate of 2min and then the resulting dry gel was maintained at 700C, 900'C, 1 0000C, 11 000C and 1 2000C forten hours, respectively. Vitrification of the dry gel was thus completed so as to produce a silica glass plate whose specific gravity was 2.20.

Carbon paper as thick as 0.3mm was laid on a graphite plate (20cm x 20cm x 1cm). The silica glass plate was mounted thereon and the assemblywas introduced into a graphite heating furnace which was heated to 1800"C. Afterthe resulting silica glass plate was maintained atthis temperature forten minutes, the silica glass plate was moved into a cold chamber and cooled to room temperature in 30 minutes. Since distortion was found in the silica glass plate, the silica glass plate was heated to 1200 C and maintained at 1200 Cforone hour and then the temperature was lowered at the rateofl00"C per hour in order to remove the distortion. The flatness thereof was accurate to less than 0.1 mm.

The resulting silica glass plate was mirror-polished into a plate of 6 inch x 6 inch x 0.12 inch, andthen collimated lightfrom a lampwas directed onto itsothatthe intensity of illumination was 50,000 lux in the dark room. However, no reflectance points were detected. When thetransmittance of ultraviolet lightthroughthe silica glass plate was measured for a wavelength of 200nm or less, the transmittance was constantly 85% or more and no specific absorption was observed.

Example 9 Awhite and porous dry gel (22cm x 22cm x 0.9cm) obtained by usingthesame method as in Example7 was placed in a glass displacementfurnace and dryairwas introduced intothefurnace ataflow rate of 2e/min. The temperature in the furnace was raised to 700"C at the rate of 60"C and was maintained atthe temperature of 7000C for 20 hours. Helium gas instead of the dry air was introduced into the furnace at a flow rate of cumin, and then the temperature in the furnace was maintained at temperatures of 800"C, 900"C and 1 0000C for five hours, respectively.When the temperature had fallen to room temperature, a white and porous dry sintering gel was obtained whose size was 18cm x 18cm x 0.7cm.

Carbon paperofathickness upto 0.3mm was laid on a graphite plate (20cm x 20cm x 1cm) andthesaid sintering gel obtained by heating to the 1000 C was mounted thereon, the assembly being placed in a graphite heating furnace. While maintaining a reduced pressure of more than 1 Torr inthefurnace by using a rotary pump, the temperature in the furnace was rapidly raised to 1 000C in ten minutes. Thetem- perature was then raised to 1300 C at a heating rate of 300"C per hour and was maintained atthetemperature of 1 3000C for one hour.At this temperature, N2 gas was introduced into the furnace continuously, thetem perature therein being raised to 1 7500C at a heating rate of 600"C per hour and maintained at the lattertem perature for 30 minutes.

The resulting transparent silica glass plate was moved into a cold chamber and chilled two room temperature in 30 minutes. The transparent silica glass plate whose size was 15.5cm x 15.5cm x 0.6cm which was so obtained had no observed fractures or cracks. Since distortion was found in the silica glass plate, the silica glass plate was heated to 1200 Cand maintained at 1200 Cforone hourandthenthetemperaturewas lowered at the rate of 1 OO"C per hour in order to remove the distortion.

The resulting silica glass plate was mirror-polished into a plate of6 inch x 6 inch x 0.12 inch andthen collimated lightfrom a lamp was directed onto it so that the intensity of illumination was 50,000 in a dark room. However, no reflectance points were detected. Optically a very high-quality silica glass plate was obtained without crystals and distortion. When the transmittance of ultraviolet lightthrough the plate was measured, for the wavelength of 200nm or more, the transmittancewas constantly 90% or more and no specific absorption was observed.

Example 10 Ten pieces of silica glass (15.5cm x 15.5cm x 0.6cm) obtained by closing pores in the dry gel on the He atmosphere according to the same method as in Example 7 were prepared. Five pieces of carbon paper whose size was 17cm x 17cm x 0.93cm and five silica glass plates were arranged alternately on a graphite plate of 20cm x z0cm x 1 cm. The graphite plate was mounted on four graphite props whose heightwas 4cm.

Further, five silica glass plates and five pieces of the carbon paper were mounted alternately thereon in the samewas as described above. In this way, the ten pieces of silica glass were set in position. Afterthefurnace atmosphere had been replaced by N2 gas, the various silica glass plates were moved into a graphite heating furnace having a temperature of 1 8000C and were maintained therein for fifteen minutes. The silica glass plates were then moved into a cold chamber and cooled to room temperature in 30 minutes. There was no fusion either between the graphite plate and the silica glass plates or between the silica plates themselves.

The flatness of each silica glass plate was accurateto less than 0.2mm. Since distortion was found in the plates, the latter were maintained at 1 2000C for one hour and then the temperature was lowered at the rate of 1 000C per hour in order to remove the distortion.

Each ofthe resulting silica glass plates was mirror-polished into a plate of 6 inch x 6 inch x 0.2 inch,and then collimated lightfrom a lamp was directed onto itso that the intensity of illumination was 50,000 Iux in the dark room. However, no reflectance points were detected. Optically very high-quality silica glass plates without crystals or distortion were obtained. When thetransmittance of ultraviolet light through the plates was measured, for a wavelength of 200nm or more, the transmittance was constantly 90% or more and no absorption was observed.

Example 71 2200ml of ethyl silicate and 1600ml of 0.02 normal hydrochloric acid solution were mixed together and stirred violently to obtain awater-whitetransparentuniform solution. 600g of ultra fine particle silica (Aerosil OX-50) was added little by littleto thewater-whitetransparent uniform solution and the solution was stirred sufficiently. This sol solution was exposed to irradiation by ultrasonic waves of 28 KHzfortwo hours ata temperature of 20"C, and after a centrifugal force of 1500 G was applied for ten minutes to remove large particle silica, the sol solution was filtered by means of a filter having holes whose diameter was 1 micron.

After the pH of the resulting solution had been adjusted to 4.8 with 0.1 normal ammonia water, the sol solution was filtered by means of a filter having holes whose diameter was 1 micron.

3770ml ofthe highly homogeneous sol solution obtained above was poured into a container (6cm inner diameter, 150cm length) such as an alurninium tube coated with TEFLON (Registered Trade Mark) and the container was sealed with a stopper. After setting the sol in the container on a revolving device and rotating the container about its central axis at a rotational speed of 500rpm for one hour, the container was left at rest forfour days at room temperature. After taking offthe stopper, the tubular gel within the containerwas removed therefrom and placed in another container (10cm x 170cm x 20cm height) made of polypropylene.

The latter container had a lid whose openings amounted to 0.5% of the surface area of the lid, the abovementioned gel being dried therein at 60 Cfor 30 days, whereby a tubular dry gel was obtained.

The said tubular dry gel was placed in a gas displacementfurnace and dry air was introduced into the latter at a flow rate of 24/mien. The temperature in the furnace was raised to 7000C at the rate of 60"C per hour and was maintained atthistemperature'orten hours.Agas mixture comprising helium (1.8Umin) and chlorine (0.2U min) was introduced into the furnace instead of the dry airtherein, and then the temperature in thefurnace was raised to 10000C at the rate of 30"C per hour. O2 gas was introduced into the furnace at a flow rate of 2Umin instead ofthe said gas mixture, and then the tubular dry gel was maintained attemperatures of 1000 C and 1050 Cforten hours, respectively.

Finally, He gas (2in) instead ofthe 2 gas was introduced into the furnace, and then the tubular dry gel was maintained attemperatures of 1050 C,1100 C and 1200 C forten hours, respectively. Although the resulting silica glass precursor member was translucent, the specific gravity thereof was almost 2.20, while the outer diameter, inner diameter and length were 3cm, 1cm and 75cm, respectively. The degree of ovality ofthe silica glass precursor member was 0.1 % and the bow was 2.0mm/m.

The silica glass precursor member so obtained above was fixed at both ends in a vertical disposition and was moved into a ring heater in which graphite was the heating element. While the argon gas was caused two flowaround the ring heater, the temperature therein was maintained at 20000C. The silica glass precursor memberwas moved so that the upper end was moved at the rate of3cm/min and the lower end was moved at the rate of 4cm/min to the vertical direction. Next, the resulting silica glass memberwas placed in an electric oven and afterthe silica glass memberwas maintained for one hour therein at a temperature of 1200 C,the temperature was lowered at a cooling rate of 100 C per hourto remove the distortion. The outer diameter, inner diameter and length thereof were 2.6cm, 0.87cm and 1 m, respectively. Although the degree of ovality was 0.1% again, the bow was improved to 0.1 mm/n.

When the silica glass memberwas exposed to irradiation by laser-lightwhose wavelength was O.6331lm, no scattering of the light was observed. Moreover, when the water content of the silica glass memberwas measured by measuring its absorption of 2.72ism wavelength light, it was then less 1 ppm.

Example 12 440mi of ethyl silicate and 360ml of 0.05 normal hydrochloric acid solution were mixed together and stirred violently to obtain a water-white transparent uniform solution. 1 SOg of the ultra fine particle silica (Aerosil OX-50) were added little by littleto the water-whitetransparent uniform solution and the solution was stirred sufficiently. This sol solution was exposed to irradiation by ultrasonic waves of 28KHzfortwo hours at a temperature of 20 C, and after a centrifugal force of 1 500G was applied for ten minutes to remove the large particle silica, the sol solution was filtered by means of a filter having holes whose diameter was 1 micron.

The pH ofthe highly homogeneoussol solution obtained above was adjusted to 4.2 with 0.1 normal ammonia water. 700ml of the resulting solution was poured into a container (30cm inner diameter, Ocm height) made of polypropylene and having a lid with openings amounting to 1%ofthesurface area ofthe lid.

When thissol solution was gelled and dried atatemperature of 60 Cforseven days, a white and porous dry gel was obtained.

The dry gel obtained above was placed in a vacuum furnace and the temperature was raised to 1 000"C atthe rate of 60"C per hour. The atmospheric pressure in the vacuum furnace was lowered to not more than 1 Torr by using a rotary pump and then, while maintaining the pressure, the temperature was raised to 1300"C atthe rate of 1 OO"C per hour. Afterthe dry gel was maintained at the temperature of 1 3000C for one hour, vitrification of the dry gel occurred. The diameter and the thickness thereof were 15cm and 0.5cm, respectively.

The silica glass plate so obtained was laid on a concave graphite member having a radius of curvature of 30 cms, and this assembly was placed in a graphite heating furnace. After the furnace atmosphere was replaced with N2 gas, the temperature was raised to 1 800"C in two hours, and thetemperature was maintained atthis temperature forten minutes. The temperature was then lowered to 1 2000C at a cooling rate of 1 0000C per hour, and the temperature was further lowered to room temperature at a cooling rate of 1 OO"C per hour. A uniform silica glass memberwhose shape was like a watch-glass was obtained. There were no blow holes and optically high quality silica glass was obtained.

Example 13 A silica glass plate (15cm diameter, 0.5cm thickness) was sintered in a vacuum furnace by the same method as in Example 1.2 and was introduced into a graphite crucible and heated therein. The silica glass platewas then placed in a graphite heating furnace having a hotpress mechanism.Afterthe furnace atmospherewas replaced by N2 gas, the temperature was raised to 1850"C in two hours and thistemperature was maintained for five minutes. After this, the silica glass plate was pressed to a pressure of 10kg/cm through graphite members, the temperature was lowered to 1 2000C at the rate of 1 0000C per hour, and was thereafter lowered to room temperature at the rate of 1 OO"C per hour.

An extremely high-quality silica glass memberwas obtained whose shape was like a crucible.

Example 14 Asilica glass plate (15.5cm x 15.5cm x 0.6cm) obtained by closing the pores in a dry gel in an Heatmosphere bythe use ofthe method ofExample7was placed in an electric furnace and maintained atatemperature of 1 600"C for 30 minutes. The temperature was lowered to 12000 at a cooling rate of 1 000"C per hour and further lowered to room temperature at a cooling rate of 100 C per hour.

When the resulting silica glass plate was mirror-polished to form a plate of 6 inch (15.24 cms) x 6 inch (15.24 cms) x 0.12 inch (0.30 cms), collimated light from a lamp was directed onto it so thatthe intensity of illumina- tion was 50,000 lux in a dark room. The silica glass plate was then examined in this lightto seewhetherthere were any inclusions in the silica glass plate which would show up in theform of points of light. Some small points of lights were detected with the naked eye in the surface.

Example 15 Asilica glass plate (15.5cm x 15.5cm x 0.6cm) obtained by closing pores in a dry gel in an Heatmosphere by the same method as Example 7 was placed in a graphite heating furnace. Afterthe furnace atmosphere was replaced by argon gas, the temperature was raised to 21 000C in two hours and was maintained atthat temperaturefor one minute. The temperature was lowered to 1 2000C at a cooling rate of 1 0000C per hour and was then further lowered to room temperature at a cooling rate of 1 OO"C per hour.

The size ofthe silica glass plate was reduced to 14cm x 14cm x 0.5cm by vaporization of silica. Thesilica glass plate was mirror-polished to form a plate of 2mm in thickness and then collimated light from a lampwas directed onto itso thatthe intensity of illumination was 50,000 lux in a dark room. However, no reflectance points were detected.

The following three Examples, namely Examples 16-18, are not within the scope of the present invention and are given for comparison purposes only.

Example 16 Asilica glass plate (15.5cm x 15.5cm x 0.6cm) obtained by closing pores in a dry gel in an He atmosphere bythe same method as Example7was placed in a furnace and maintained atatemperature of 1450"Cfor30 minutes. When the temperature was lowered to room temperature, the surface of the silica glass plate became whit bycrystalization.

When the resulting silica glass plate was mirror-polished to form a plate of 6 inch (15.24cms) x 6 inch (15.24 cms) x 0.12 inch (0.30 cms) collimated lightwas directed onto itso thatthe intensity of illuminationwas 50,000 lux in a dark room, a light spot was clearly visible. Moreover, reflectance points of various sizeswere visible in the glass.

Example 17 Asilica glass plate (15.5cm x 15.5cm x 0.6cm) obtained by closing pores in a dry gel bythesame method as in Example 7 was placed in a graphite heating furnace. After the furnace atmosphere had been replaced with argon gas, the temperature was raised to 2300 C rapidly and then lowered to room temperature. There was little silica glass left in thefurnace.

Example 18 Awhite and porous dry gel obtained by drying by the same method as in Example 4was heated to 1300"C in the atmosphere so that a transparent silica glass plate was obtained whose size was 10cm x 10cm x 0.5cm.

Inclusions whose diameter was about 10 microns and blow holes were detected in the plate. The silica glass plate was placed in a graphite heating furnace. After this, the furnace atmosphere was replaced with N2 gas and maintained at a temperature of 1800"Cforten minutes. Sinceviolent bubbling occurred in the silica glass, the volume of the glass was increased threefold.

As mentioned above the present invention provides an improved silica glass member so far as its optical quality is concerned,the glass or glass precursor preferably being heated to 1 5000C to 2000"C and maintained at this temperature for a predetermined period of time in the sol-gel method.

It is desirable to close the pores in the dry gel in an helium atmosphere under the reduced pressure in order to prevent bubbling. However,the present invention is not limited to a particular method of preparing thesol or a particular method of heating. Furthermore, this invention can be applied to articles ofvarious shapes.

The present invention enables even silica glass made by the sol-gel method to be used not onlyforthe silica substrate of an IC Mask of a support tube for optical communication fibres but also to be used forthe mother rod for optical communication fibres, and soon.

Claims (24)

1. A method of preparing a silica glass member, comprising the steps of: drying a sol solution containing a silica compound so as to form a drygel, obtaining a glass ora glass precursor member by closing pores in said drygel,and obtaining a silica glass member by heating said glass or glass precursor memberto a selected temperature substantially in the range between 1500 and 2200"C a nd maintaining said glass orglass precursor at said temperature for a predetermined period oftime.

2. A method as claimed in claim 1, wherein said sol solution is obtained by hydrolyzing alkyl silicate with water and with an acid or basic reagent.

3. A method as claimed in claim 1,wherein said sol solution is obtained by mixing together at a predetermined mixing ratio a solution of alkyl silicate hydrolyzed with an acid reagent and a solution including particulate silica obtained by hydrolyzing alkyl silicate with a basic reagent.

4. A method as claimed in claim 1,wherein said sol solution is obtained by mixing together at a predetermined mixing ratio a solution of alkyl silicate hydrolyzed with an acid reagent and particulate silica.

5. A method as claimed in claim 1, wherein said so solution is obtained by diffusion particulate silica into water or into an organic solvent at a predetermined ratio.

6. A method as claimed in any of claims 3-6 in which the particulate silica has a mean particle diameter between 0.01 and 1.0 microns.

7. A method as claimed in any preceding claim, wherein the pores in said dry gel are closed bysintering said dry gel in an He atmosphere.

8. A method as claimed in any of claims 1-6 wherein the pores in said dry gel are closed by sintering the dry gel under a reduced pressure.

9. A method as claimed in any of claims 1-6, wherein the pores in said dry gel are closed by sintering the dry gel under a reduced pressure after the dry gel has been processed in an He atmosphere.

10. A method as claimed in any preceding claim, wherein a gas burner is used for heating said glass or glass precursor member to the said selected temperature.

11. A method as claimed in any of claims 1-9, wherein the glass or glass precursor member is heated to the said selected temperature by means of a high temperature furnace having a graphite, a tungsten or a molybdenum heating element.

12. A method as claimed in any of claims 1-9, wherein the glass or glass precursor member is heated to the said selected temperature by means of a high temperature continuous heat-treating furnace.

13. A method as claimed in any of claims 1-9, wherein the glass or glass precursor member is heated to the said selected temperature by means of a high temperature gas furnace in which the combustion of hydrogen ora hydrocarbon gas is used as a heat source.

14. A method as claimed in claim 11 or 12, wherein spacer means are provided between the glass or glass precursor member and a supporttherefor.

15. A method as claimed in claim 14, wherein said spacer means is constituted by carbon powder, carbon fibres or paper-like orfabric-like material obtained by processing carbon fibres.

16. A method as claimed in claim 14wherein said spacer means is constituted by a powder which is hard to sinter.

17. A method as claimed in claim 15 in which the said powder is alumina, zirconia, or silicon nitride.

18. A method as claimed in any of claims 14to 17 wherein a number of said glass orglassprecursor members are simultaneously heat-treated while mounted on said spacer means.

19. A method as claimed in any preceding claim, wherein said glass or glass precursormemberis mounted by using a casting of desired configuration when said glass or glass precursor member is heated to the said selected temperature.

20. Amethod as claimed in any of claims 1-18wherein said glass orglass precursor member is moulded to a desired configuration by subjecting said member to an external force when said glass or glass precursor member is heated to the said selected temperature.

21. A method as claimed in any preceding claim,wherein, afterthe glass or glass precursor memberhas been heated to the said selected temperature, it is cooled to room temperature in a plurality of stages, the cooling rate being smaller in at least one of said stages than in another stage or stages.

22. A method of preparing a silica glass member substantially as described in any ofthe Examples 1-15.

23. A silica glass member when made by the method of any preceding claim.

24. Any novel integer or step, or combination of integers or steps, hereinbefore described and/orshown in the accompanying drawings irrespective of whetherthe present claim is within the scope of, or relatesto the same or a different invention from that of, the preceding claims.