JPH042625A - Production of highly homogeneous silica glass - Google Patents

Abstract

PURPOSE:To obtain high-purity silica glass having homogeneous optical characteristics in good yield by dispersing fine silica powder in pure water, etc., forming the glass from the resultant slurry, then carrying out sintering and hot pressing treatment of the formed compact in a specific state. CONSTITUTION:(a) Fine silica powder is dispersed in pure water or a mixed solution of the pure water and hydrofluoric acid, ammonium fluoride, ammonia, an alcohol or acetic acid to prepare a slurry, which is then filtered and dried to form the fine silica powder. (b) The aforementioned formed compact is then charged into helium gas or a mixed gas atmosphere of the helium gas and chlorine at >=1450 deg.C and sintered. (c) Hot pressing treatment for applying >=50 kgf/cm<2> pressure from a uniaxial direction to the above-mentioned sintered compact at >=1650 deg.C is subsequently carried out to afford the objective silica glass. The fine particles herein described are powder having a primary particle diameter of <= the order of several mu.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing highly homogeneous optical silica glass. In particular, (a) a silica glass powder molded body is formed by filtration molding using silica fine powder as a raw material, and (b)
The molded body is sintered at a temperature of 1450°C or higher, and (c
This invention relates to a method for obtaining a desired highly homogeneous silica glass by subjecting a sintered body to a hot press (hereinafter referred to as HP) treatment.

[Prior Art] Recently, in the semiconductor field, the development of equipment using lasers, such as reduction projection exposure equipment and thin film manufacturing equipment for CVD, is progressing, and due to the demand for more integrated equipment, the wavelength of the laser used is also becoming shorter. wavelength. Silica glass, which has excellent transmittance in the ultraviolet region, is used as the optical glass, and a glass member with a homogeneous refractive index suitable for precise control of the optical system and design of the optical system is desired.

Conventional silica glass includes fused natural quartz and synthetic silica glass made from silicon tetrachloride, each of which is highly homogeneous, but it is necessary to remove impurities, striae, and low homogeneity from the raw materials. It is not optically or industrially satisfactory due to yield problems caused by this.

In addition, as a recent silica glass manufacturing method, there is a sol-gel method, which is being actively researched. There are many problems with industrialization, such as the problem of.

[Problems to be solved by the invention] The present invention uses fine silica powder as a raw material, and processes such as powder molding, sintering,
The aim is to provide high-purity silica glass with uniform optical properties at a high yield through a high-temperature, high-pressure treatment process.

[Means for Solving the Problems] The present inventors have completed the present invention as a result of intensive studies to solve the above problems.

That is, the present invention includes: (a) preparing a slurry by dispersing fine silica powder in pure water or a mixed solution of pure water and hydrofluoric acid, ammonium fluoride, ammonia, alcohol, or acetic acid; filtering the slurry; a step of molding fine silica powder by drying, (b) introducing the molded body into an atmosphere of helium gas or a mixed gas of helium and chlorine at a temperature of 1450° C. or higher;
sintering step, and (e) applying 50 kg to the sintered body at a temperature of 1650°C or higher;
This is a method for producing highly homogeneous silica glass characterized by comprising a step of performing a hot press treatment in which a pressure of f/cgff or more is applied from one axis. The present invention will be explained in more detail below.

In producing the silica glass of the present invention, the starting material was silica fine powder. The fine powder here refers to powder with a particle diameter of several microns or less.

Fine powder can be obtained by the sol-gel method, soot method, etc., but it is important to reduce impurities and the particle size of the powder.

From the viewpoint of controlling the surface area, it is preferable to obtain it by hydrolyzing a silicon alkoxide such as tetraethoxysilane.

There are generally dry and wet molding methods, and in the present invention, filtration molding, which is one of the wet molding methods, is used.

Since the filtration molding method uses a slurry of fine silica powder, the powder is dispersed until the time of molding, and unlike mechanical presses, pressure is applied to the entire body, resulting in a molded product with a uniform structure and uniform bulk density. This is because it becomes possible to do so. Further, during molding, the powders cause necking, and a molded body having a certain degree of strength is produced while leaving open holes. In order to give further strength to the molded body produced by the above method, there is no problem in applying cold hydrostatic pressure.

A molded body formed by filtration molding has a significantly smaller amount of foaming after sintering than a molded body formed by dry molding using a mechanical press, etc., and provides glass with a uniform density even after sintering. Moreover, unlike the sol-gel method and slip casting, the filtration molding method of the present invention removes a certain amount of water in the molded product through filtration, making it possible to shorten the drying time compared to other wet molding methods. Ta. Furthermore, removal of water by filtration avoids cracking during drying, so it is possible to increase the size.

The slurry is prepared by dispersing fine silica powder in pure water or a mixture of pure water and hydrofluoric acid, ammonium fluoride, acetic acid, ammonia, or alcohol. Ethyl alcohol is preferred as the alcohol. The structure of the secondary particles differs depending on these solvents, and the density and
Since this will result in a difference in strength, the mixing ratio of the solvents may be changed as necessary. The dispersion method may be any method such as ultrasonic dispersion or ball mill dispersion. The slurry concentration may be any concentration as long as it has a viscosity that allows filtration, but it is preferably a concentration that does not cause gelation due to the thixotropic effect of the fine powder during filtration.

The slurry may be filtered by any method such as suction filtration or pressure filtration. Molding is performed simultaneously with this filtration.

There are no particular limitations on the method, but for example, a mold without a bottom may be placed on a filter, slurry may be poured into it, and filtration and molding may be performed at the same time. The filtration time is continued until the molded body on the filter can maintain its shape, specifically, until the moisture on the surface approximately disappears. The molded body can be changed in this way.

The drying process may be carried out in a desiccator or in a dryer, but preferably in a humidifier to prevent cracking in the early stages of drying.

Next, sintering is performed, but prior to that, the molded body is
Preferably, the chlorination is carried out at a temperature below 450°C.

This allows dehydration of the molded body and removal of metal impurities.

The sintering step is performed at a temperature of 1450° C. or higher, at which point the molded body becomes vitrified. Any sintering method may be used as long as the atmosphere is helium or a mixed gas of helium and chlorine during vitrification. For example, in order to obtain glass with fewer residual bubbles, 10 +
It is preferable to vitrify the molded body from the end by inserting it at a speed of less than am/min.

In the final step of HP treatment, the temperature is raised to 1650° C. or higher to completely complete the sintering of the powder and rearrange the glass structure to provide a uniform and nearly perfect glass structure. At the same time, striae and residual air bubbles can be removed using a pressure of 50 kgf/cm2 or higher, making it possible to produce striae-free, bubble-free, highly homogeneous silica glass. In addition, compared to hot isostatic pressing, HP processing prevents the molded product from deforming, makes it easier to finish into the desired shape, and because no gas is used, the gas that has melted into the molded product foams when reheated. It has the advantage that there is no fear of The HP treatment atmosphere may be any atmosphere such as argon gas, nitrogen gas, or a mixed gas of oxygen and argon, and it is preferable that the pressure be raised and lowered at a sample temperature of 1200° C. or lower. Although the sintered body as a sample may be left as is, it is preferably wrapped in metal foil such as molybdenum metal foil or platinum foil in order to maintain the initial shape of the sintered body.

[Examples] In order to explain the present invention in more detail, Examples are given below, but the present invention is not limited thereto.

In addition, the homogeneity measurement shown below was made using #140 glass after glass production.
After finishing with a surface grinder and annealing in the atmosphere at 1150° C., the refractive index difference was measured using interferometer method with light having a wavelength of 632.8 nm.

[Example 1] a) Preparation of fine silica powder Silica fine powder was prepared by decomposing silicon tetrachloride with an oxyhydrogen flame and having an average particle size (-particle size) of 50 nm and a surface area of 50 m.
2/g powder was prepared.

b) Preparation of silica slurry Mix powder and pure water at a weight ratio of 10:1, apply ultrasonic waves for 1.5 hours, disperse the powder in pure water, and pass through a 10 μm mesh to form a slurry. used.

C) Preparation of molded body Filter the slurry using a suction filtration method using a 90 mmφ vinyl chloride mold and a filter paper with a pore size of 1.0 μm.
Molded. Filtration was performed until the moisture on the surface disappeared, and then the molded body was dried for one week in a constant temperature and humidity chamber at a humidity of 90% and a temperature of 60°C.

The obtained molded body has a diameter of 90 mmφ, a thickness of 20 mm, and a density of 0.
．． It was 8 g/cm'.

d) Sintering The sintering process is divided into two steps: chlorination and vitrification. First, chlorine treatment is performed at 700°C to 1000°C, then replaced with helium gas, and the temperature is raised to 1300°C.
The molded body was held for a period of time to shrink, and then moved to the hearth.

Thereafter, the temperature inside the furnace was raised to 1500° C., and while the temperature was maintained, the molded body was inserted into the furnace at a speed of 10 mm/min over 0.5 hours to vitrify it.

e) High temperature and high pressure treatment The above sintered body was wrapped in metal molybdenum foil and loaded into a hot press device. Note that graphite powder was used as the filling powder.

The temperature was raised to 1700°C and held for 1 hour, and then heated to 115°C.
Let the temperature drop naturally to 0℃ and hold for 1 hour, then 200℃
/hr to lower the temperature to room temperature. On the other hand, the pressure is 250 kg from uniaxial direction above and below 1000℃ when the temperature rises.
Compressed at a pressure of f / cm 2. The pressure was released at 1200° C. when the temperature was lowered, and thereafter the pressure was set to atmospheric pressure.

Table 1 shows the measurement results of the uniformity of the silica glass obtained through the above steps.

[Example 2 a) Preparation of fine silica powder Tetraethoxysilane, ethanol, pure water.

28% ammonia water in a weight ratio of 1:2:0.7:
Mix at a ratio of 0.2, filter, dry, and dry in an oxygen stream.
Calcined at 00℃, average particle size (-charcoal particle size) 0.8μ
A powder with a m2 surface area of 5 m2/g was obtained.

b) Preparation of Silica Slurry Powder and pure water were mixed at a weight ratio of 10:1, and ball milling was performed for 1 hour using nylon balls to disperse the powder in pure water. The slurry was passed through a 10 μm mesh and used as a mesh bath.

C) Preparation of molded body Filter the slurry using a suction filtration method using a 90 mmφ vinyl chloride mold and a filter paper with a pore size of 0.3 μm.
Molded. Filtration was performed until the moisture on the surface disappeared, and then the molded product was dried for one week in a constant temperature and humidity chamber at a humidity of 90% and a temperature of 60° C., and then a cold hydrostatic pressure of 0.5 t was applied. The obtained molded body has a diameter of 90 mmφ, a thickness of 20 mm, and a density of 0.
．． 8 g/cm3 d) Sintering The sintering process is divided into two steps: chlorination and vitrification. First, chlorine treatment was performed at 700° C. to 1000° C., followed by replacement with helium gas, and the temperature inside the furnace was raised to 1500° C. to vitrify it.

e) High temperature and high pressure treatment The above sintered body was wrapped in metal molybdenum foil and loaded into a hot press device. Note that graphite powder was used as a filler.

The temperature was raised to 1750°C, held for 1 hour, and then lowered. The temperature lowering conditions are the same as in Example 1. On the other hand, the pressure was compressed at 200 kgf/cm 2 from 1000° C. in uniaxial directions above and below when the temperature was rising, and was released at 1200° C. when the temperature was falling, and then the pressure was set to atmospheric pressure.

Table 1 shows the measurement results of the uniformity of the silica glass obtained through the above steps.

[Example 3 a) Preparation of fine silica powder The powder of [Example] was used.

b) Preparation of silica slurry Powder and 0.02M hydrofluoric acid were mixed at a weight ratio of 5:1, stirred for 24 hours, and the powder was dispersed in pure water. The slurry was passed through a 10 μm mesh and used as a mesh bath.

C) Preparation of molded body It was molded in the same manner as in Example 1. The density of the obtained molded body was 0.7 g/cm3.

d) Sintering The mold was sintered in the same manner as in Example 1, except that the moving speed of the mold was 5 mm/min and the mold was inserted to the center of the furnace over a period of one hour.

e) High temperature and high pressure treatment The above sintered body was wrapped in platinum foil and loaded into a hot press device. Note that graphite powder was used as a filler.

The temperature was raised to 1650°C, held for 1 hour, and allowed to cool down naturally. On the other hand, the pressure is compressed at a pressure of 250 kgf/cm2 from uniaxial direction above and below 1000℃ when the temperature rises,
When the temperature decreased, the pressure was removed at 1200°C, and then the pressure was set to atmospheric pressure. The temperature lowering conditions were the same as in Example 1.

Table 1 shows the measurement results of the uniformity of the silica glass obtained through the above steps.

[Comparative Example] Using the powder of Example 2, it was molded by cold isostatic pressing, hot pressed under the conditions of Example 1, and the diameter was 50 cm.
A glass having a thickness of 20 fflI11 was obtained.

Table 1 shows the measurement results of the uniformity of the silica glass obtained through the above steps.

table As is clear from Table 1, Manufactured by the method of the present invention The glass that has been Exceptionally superior optical homogeneity It was.

Claims (1)

[Claims] In a method for producing silica glass, (a) silica fine powder is dispersed in pure water or a mixed solution of pure water and hydrofluoric acid, ammonium fluoride, ammonia, alcohol, or acetic acid to prepare a slurry. and molding fine silica powder by filtering and drying the slurry, (b) introducing the molded body into an atmosphere of helium gas or a mixed gas of helium and chlorine at a temperature of 1450 ° C. or higher,
sintering step, and (c) applying 50 kg to the sintered body at a temperature of 1650°C or higher;
A method for producing highly homogeneous silica glass, comprising a step of hot pressing in which a pressure of f/cm^2 or more is applied from one axis.