JPH03232728A - Production of silica glass - Google Patents

Abstract

PURPOSE:To enable production of a large-size monolithic silica glass free from defects such as an air bubble by calcining a dry gel from one direction in order when producing the subject silica glass using sol-gel process. CONSTITUTION:A sol obtained by hydrolysis of a silicon alkoxide, its polycondensation product, etc., is gelled while keeping the sol at room temperatures-70 deg.C and the resultant gel is dried at >= room temperatures for several weeks to prepare a dry gel. The obtained dry gel is heated at <=burning temperature and >= organic component-decomposition temperature, ordinarily 400-1000 deg.C in an oxidative atmosphere in order to oxidize or heat decompose organic components in the dry gel and to remove the components. The dry gel 4 is then put in a unit 5, sent to a furnace 1 and burnt (burning temperature: 1000-1600 deg.C, preferably temperature gradient: >=100 deg.C/cm, moving speed: <=2cm/h) from one direction in order, thus obtaining the objective silica glass 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing silica glass used for optical, semiconductor, electronic, physical and chemical applications, etc.

[Prior art] Silica glass has excellent heat resistance, corrosion resistance, and optical properties, so it is an important material indispensable for semiconductor manufacturing, and is also used as a material for optical fibers, photomask substrates for IC manufacturing, and TPT. It is used for circuit boards, etc., and its applications are expanding more and more.

Conventional methods for manufacturing silica glass include melting natural quartz in an electric furnace or oxyhydrogen flame, or oxidizing and melting silicon tetrachloride at high temperature in an oxyhydrogen flame or plasma flame. However, the manufacturing process requires high temperatures of 2000°C or higher, which consumes a large amount of energy.In addition, materials that can withstand such high temperatures are required during manufacturing, and it is difficult to obtain high-purity materials, making it economically difficult. , there are some quality problems.

On the other hand, in recent years, a method of synthesizing silica glass at low temperature called the sol-gel method has been attracting attention.

The outline will be briefly described below.

General formula S i (OR) 4 (R: alkyl group)
Add water (pH may be adjusted with alkali or acid) to silicon alkoxide represented by and hydrolyze it to form silica hydrosil (referred to as silica sol in the present invention).
shall be. At this time, an appropriate organic solvent such as alcohol is generally added as a solvent so that the silicon alkoxide and water form a uniform system. This silica sol is gelled by standing still, increasing the temperature, adding a gelling agent, etc. Thereafter, the gel is dried to obtain a dry gel. Silica glass is obtained by firing this dried gel in a suitable atmosphere.

[Problems to be Solved by the Invention] However, there are still unresolved problems in the production of silica glass by the sol-gel method. Since dried gel is a porous material, air bubbles are easily trapped in the sintered body during the firing process, making it difficult to produce monolithic large-sized silica glass without defects such as air bubbles using the sol-gel method. It is.

The present invention provides a method for producing large monolithic silica glass free of defects such as bubbles by a sol-gel method.

[Means for Solving the Problems] The present invention provides a method for producing silica glass in which silicon alkoxide is hydrolyzed to form a silica sol, this is gelled, dried to form a dry gel, and then fired, in which the dry gel is fired. The present invention relates to a method for producing silica glass characterized by sequentially firing a dried gel from one direction in the steps.

The silicon alkoxide in the present invention has the general formula S i
In addition to those represented by (OR) 4 (R: alkyl group), polycondensates thereof, such as (RO)3S i ・
(O8i (OR) 2) n・O8i (OR)
(n=0-8, R-nialkyl group) is also used.

As the alkyl group of silicon alkoxide, a methyl group, ethyl group, propyl group, or butyl group is used from the viewpoint of ease of hydrolysis and gelation time.

As the solvent to be added together with water, a base is preferable to an acid in terms of gelation time and ease of sintering the resulting dry gel.

As the organic solvent such as alcohol added together with water, alcohols such as methyl alcohol, ethyl alcohol, 1-propyl alcohol, 2-propyl alcohol, acetone, dimethylformamide, etc. are used from the viewpoint of solubility in both water and alkoxide. It will be done.

The silicon alkoxide, organic solvent, and water are thoroughly mixed using a Stark or the like to make the resulting gel as uniform as possible. Alternatively, ultrasonic waves may be irradiated. Fine particles of silica or organic polymer compounds such as polyvinyl acetate, polyalkylene glycol, polyalkylene glycol ether, and hydroxyalkyl cellulose may be added during sol preparation.

The silica sol is transferred to a container, kept at room temperature to 70°C to gel, and dried at a temperature above room temperature for several weeks to form a dry gel.

The dried gel obtained in this way is then kept in advance at a temperature below the sintering temperature and above a temperature at which the organic components contained in the dried gel are decomposed, so that the organic components contained in the dried gel, i.e., the raw material silicon alkoxide, are removed. It is preferable to decompose and remove organic components such as organic components resulting from alkyl groups therein, organic polymer compounds such as polyvinyl acetate and polyethylene glycol, and organic solvents.

Here, below the sintering temperature and above the temperature at which organic components are decomposed is usually 400°C to 1000°C. The atmosphere during the decomposition treatment is preferably an oxidizing atmosphere such as air or oxygen gas. Through this treatment, organic components in the dried gel undergo oxidation or thermal decomposition and are removed. By removing the organic components, the risk that defects such as free carbon and air bubbles will be trapped in the sintered body during the next firing process is further reduced.

The dried gel thus obtained is sent to a firing furnace, and the dried gel is sequentially fired from one direction. FIG. 1 shows an example of a method for manufacturing such silica glass, in which 1 is a firing furnace, 2 is a heater, 3 is fired silica glass,
4 is a dry gel, and 5 is a jig for supporting the dry gel.

Here, firing the dried gel sequentially from one direction means firing the dried gel sequentially from one end while moving the dried gel in the firing furnace. At this time, the heating source may be moved instead of moving the dry gel. The direction in which the dry gel or the heat source is moved is not particularly limited, but it may generally be moved vertically or horizontally.

The firing temperature is the firing temperature generally used in the production method of silica glass by the sol-gel method, that is, 1000 ~
The temperature is 1600°C.

Regarding the temperature distribution inside the kiln, it is best to keep it uniform in the plane perpendicular to the kiln wall, that is, in the plane perpendicular to the moving direction of the dried gel, and to make it uniform in the plane parallel to the kiln wall. 20°C/cm or more, preferably 50°C along the direction of the dried gel to be fired, from the central point of the heater heated to the highest temperature in the furnace in the direction parallel to the moving direction of the dried gel. /cm or more, most preferably 100℃/
It is preferable to provide a temperature gradient of Cm or more.

Dry gel, which is a porous body, contains many open pores, and at the stage of making it non-porous by the sintering method, the sintered body area that has been made non-porous and the unsintered body where open pores remain. There is a transition region between these regions, and the open pores in the dry gel become closed pores as sintering progresses, but the gas trapped in the closed pores moves into the transition region as sintering progresses. It is thought that the particles diffuse and move to the region of the unsintered body where open pores remain, and the closed pores disappear. Therefore, it is considered that the larger the temperature gradient is during sintering, the narrower the width of the transition region becomes.In other words, the diffusion distance of gas in the closed pores becomes shorter, and no air bubbles remain in the sintered body.

Preferably, the temperature gradient mentioned above is maintained at least in the transition region.

The speed of movement of the dried gel to the firing furnace is appropriately selected depending on the above-mentioned temperature gradient of the firing furnace, but it is usually at a speed of 5 cm/h or less, preferably 2 cm/h or less.

The temperature gradient, the calcination temperature, and the movement speed of the dried gel each influence each other. The larger the temperature gradient, the lower the movement speed of the dry gel, and in order to increase the movement speed of the dry gel, the firing rate must be increased. The larger the temperature gradient and the smaller the movement speed of the dried gel, the higher the yield.

The atmosphere in the firing furnace is such that air bubbles are not trapped in the silica glass and can easily escape from the system.

The most preferable such atmosphere is a He gas atmosphere. Depending on the firing conditions, a gas atmosphere such as Ar, N2.02, or air, or a mixed gas atmosphere of these gases and He gas may be used.

The above-mentioned firing furnace may be a tunnel furnace or the like having a chamber divided into an organic component decomposition section and a firing section, and can continuously perform the decomposition of the organic components and the firing of the dried gel. In addition, although the firing furnace only has a firing section, it is possible to decompose the organic components and fire the dry gel continuously by appropriately adjusting the firing temperature settings, the moving speed of the dry gel, etc. You can.

The present invention will be explained in more detail below with reference to Examples.

[Example] 90 g of water, 200 g of methyl alcohol, 0.07 choline
25 g of polyvinyl acetate was added thereto and dissolved. The obtained solution was slowly added to 175 g of a polycondensate of silicon methoxide (weight average molecular weight: 360 to 470) and mixed to obtain a silica sol. This was coated with polyfluoroethylene to a diameter of 62 mm and a depth of 2.
It was placed in a 50 mm glass cylindrical container, covered with aluminum foil, and gelatinized at room temperature. Thereafter, a hole was made in the lid, and the gel was dried for two weeks in a thermostat at 50° C., and then transferred to a thermostatic oven at 120° C. and dried for one day to obtain a dried gel with a diameter of 50 mm and a length of 200 mm.

The obtained dried gel was heated from room temperature to 800° C. in air at a heating rate of 50° C./h to oxidize or thermally decompose the organic matter in the dried gel. As shown in Figure 1, the atmosphere is He gas, the maximum temperature inside the furnace is heated to 1450°C, and the direction is parallel to the wall of the firing furnace from the central point of the heater heated to 1450'C. about 150℃/cm along
Using a quartz jig, the gel was inserted from the bottom into a firing furnace with a temperature gradient of 1.0 cm/h, and while moving upward at a temperature of 1.0 cm/h, the gel was sintered and vitrified sequentially from the top end. . As a result, a cylindrical transparent silica glass with a diameter of 26 mm and a length of 100 mm and no bubbles inside was obtained.

[Effects of the Invention] According to the present invention, large-sized silica glass without defects such as bubbles can be easily produced by a sol-gel method.

Furthermore, according to the present invention, silica glass can be manufactured at a lower cost than before, so it can be applied not only to fields such as photomask substrates for IC manufacturing, which have been conventionally used, but also to substrates for liquid crystal displays, etc.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a method for manufacturing silica glass according to the present invention. 1... Firing furnace 2... Heating heater 3.
・・Fired silica glass

Claims (1)

[Claims] 1. In the method for manufacturing silica glass in which silicon alkoxide is hydrolyzed to produce silica sol, this is gelled, dried to form a dry gel, and then fired, the dry gel is fired sequentially from one direction in the step of firing the dry gel. A method for producing silica glass characterized by: