JPH0238325A - Production of silica glass - Google Patents

Abstract

PURPOSE:To obtain a silica glass prevented from crack or fissure development by gelling a sol solution prepared by hydrolyzing a silicon alkoxide incorporated with N-methyl-pyrrolidone and silica fine granules, followed by drying and calcination. CONSTITUTION:A silicon alkoxide or its polycondensate is incorporated with 0.1-5.0 molar times of N-methyl-pyrrolidone and silica fine granules and the resultant system is homogeneously dispersed in the presence of a solvent followed by addition of water in the presence of a catalyst to make a hydrolysis, thus preparing a sol solution. Thence this solution is gelled at >=0 deg.C and the resultant gel is left to stand at 0-100 deg.C for several minutes to several tens days to effect drying followed by calcination by heating at a rate of 50-200 deg.C/hr to 1000-1300 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing silica glass used for optics, semiconductor industry, electronic industry, physics and chemistry, etc.

(Prior Art) In recent years, the sol-gel method has been attracting attention as a new method for producing silica glass.

The method for producing silica glass by the sol-gel method is as follows.

Silicon alkoxides and/or polycondensates thereof represented by the general formula 5i(OR)n (where R represents an alkyl group), such as (RO)isi(OSi(OR))nO5i(OR)+
(n = O~8) with water (acid as a catalyst in advance,
(Optionally, an alkali may be added in advance) and hydrolyze it to form silica hydrosil. At this time, an appropriate alcohol may be added as a solvent so that the silicon alkoxide and water form a homogeneous system. Let this silica sol solution stand still, raise the temperature,
It is gelled by adding a gelling agent or the like.

Thereafter, the gel is dried to obtain silica gel.

Silica glass is obtained by sintering this dried gel in a suitable atmosphere.

This sol-gel method has the following features.

(Since it can be produced at a lower temperature than the conventional vapor phase chemical vapor deposition method in which the soot produced by flame hydrolysis etc. using 115iC14 etc. as a raw material is sintered and vitrified, it can save energy and reduce costs.

(2) Since it can be used as a raw material in a liquid state, it can be easily purified and highly purified.6 (3) It can be mixed in a liquid phase at room temperature, so it can form a homogeneous glass even when mixed with other components.

There are still unresolved problems in the production of silica glass by the sol-gel method, which has various advantages as described above.

In particular, the problem is that cracks and fractures are likely to occur in the gel during the process of drying the gel, making it difficult to produce a monolithic large-sized dried gel with a good yield. One of the causes of cracks and cracks is that stress is generated in the gel as water, alcohol, etc. evaporate during gel drying, and it is thought that if this stress is greater than the strength of the gel, the gel will crack or crack. There is. Further, this stress increases as the surface tension increases and as the pore diameter decreases, as expressed by the following formula % (ΔP - stress, γ - surface tension, θ = wetting angle, r = pore diameter).

Therefore, as a method to prevent such cracks and cranks, measures have been taken such as increasing the amount of water in the sol solution to increase the gel strength, and increasing the hydrolysis temperature to increase the pore diameter. There is.

In addition, JP-A-61-183129 discloses that solvents with boiling points higher than water, such as 1-
A method of adding butanol, 1-pentanol, 1-hexanol, and toluene to silicon alkoxide has been described.

(Problems to be Solved by the Invention) However, in all of the above methods, it was difficult to obtain a sufficiently large dry gel body, and it was not possible to obtain a silica glass body whose dimensions were very large.

The present invention provides the production of silica glass that does not generate cracks or breaks.

(Means for Solving the Problems) The present invention considers that stress generated due to evaporation of water, alcohol, etc. during gel drying is a cause of cracks and cracks, and the present invention has been developed to prevent the occurrence of such large stress. In order to avoid this problem, N-methyl-pyrrolidone is used as the silicon alkoxide medium and silica fine particles are added to the gel.

As the silicon alkoxide used in the present invention, not only silicon alkoxide monomers but also polycondensates may be used, for example, (CH30) isi (O5i (OCHs) z) n
O3i(OCI+3)3(n=O~8) can be mentioned. One or more of these can be used. As the alkyl group of silicon alkoxide, methyl group, ethyl group, propyl group, butyl group, etc. are preferable.

To the water added for hydrolysis, mineral acids such as hydrochloric acid and nitric acid, organic acids such as formic acid and acetic acid as catalysts, and not only ammonia but also organic bases such as ethylenediamine as bases may be added in advance.

The amount of N-methyl-pyrrolidone added to silicon alkoxide is 0.1 to 5% based on silicon alkoxide.
．． 0 times (molar ratio) is preferred. Moreover, commonly used alcohols such as ethanol can also be used together. Furthermore, ketones and esters can also be used in combination.

The silica fine particles are uniformly dispersed in a solvent or a mixed solution of a solvent and silicon alkoxide before being hydrolyzed by adding water. At this time, it is effective to use a surfactant to improve dispersibility.

The silicon alkoxide, N-methyl-pyrrolidone, silica fine particles, and water are thoroughly mixed using a stirrer or the like to make the resulting sol as uniform as possible. Alternatively, ultrasonic waves may be irradiated.

The generated sol solution is quickly transferred to another container and allowed to gel. During gelation, it is preferable to seal the container to prevent the solvent from escaping from the generated gel, and the temperature during gelation is preferably 0° C. or higher.

In the drying process, instead of using a lid with holes, the gel is dried and shrunk and solidified in an appropriate atmosphere to form a dry gel.

Silica glass is then produced by sintering using the gel-sol method.

Generally used conditions are used for the gelling step, drying step, and sintering step. For example, O'C
Leave at ~100°C for several minutes to several 10 days, leave at room temperature ~100°C for several hours to several 10 days, and leave at room temperature ~100°C for 1000 to 13 days in an appropriate atmosphere.
For example, heating to 00°C at a temperature increase rate of 50 to 200°C/hour.

(Function) The details of the action of N-methyl-pyrrolidone and silica particles are unknown, but the formation of silica particles in the gel, the bonding between these silica particles in the gel, and the formation of silica particles in the gel during the drying process. It is thought that this contributes to stress relaxation and the like, making it possible to increase the size of the gel.

Example I After adding silica fine particles having a particle size of about 7001 to N-methyl-pyrrolidone at a weight ratio of 0.1 times the weight ratio of morpholine and uniformly dispersing the particles, silica particles having a particle size of about 7001 mm were added to N-methyl-pyrrolidone and dispersed uniformly. Methoxysilane was mixed to create a homogeneous solution, and 0.01 mo171 aqueous solution of choline was added in a weight ratio of 0.64 times that of tetramethoxysilane, and thoroughly mixed to obtain a silica sol.

The obtained sol was placed in a stainless steel Petri dish with a diameter of 200 m to a depth of 100 m, sealed, and allowed to gel at room temperature for 5 days. Thereafter, it was dried at 60° C. for 7 days and further dried at 120° C. for 1 day to obtain a dried gel with a diameter of about 170 mm. The bulk density of the dry gel thus obtained was 0.65 g/cd, with no cracks or breaks. This dried gel was heated in air to 1250° C. at a rate of 60° C./hour to obtain silica glass having a diameter of about 110 mm without cracks or foaming. This silica glass has no devitrification or bubbles and is of high quality. Further, as a result of analysis, the properties of this silica glass matched those of commercially available silica glass.

Example 2 A dry gel having a diameter of about 170 mm was obtained in the same manner as in Example 1, except that a mixed solvent having a volume ratio of methanol:N-methyl-pyrrolidone=7:3 was used. The dried gel thus obtained was free of cracks and cracks. This dried gel was heated in the same manner as in Example 1 to obtain silica glass having a diameter of about 110 mm without cracks or foaming. This silica glass has no devitrification or bubbles and is of high quality. Further, as a result of analysis, the properties of this silica glass matched those of commercially available silica glass.

(Effects of the Invention) According to the present invention, large-sized silica glass can be easily manufactured by the sol-gel method without generating cracks or breaks. There are basically no restrictions on its size, and its shape is not limited to plate-like ones, but also rod-like and tubular ones, which can be manufactured at a lower cost than in the past.

In addition, since silica glass can be manufactured at a lower cost than before, it can be used not only in fields such as photomask substrates for IC manufacturing, which have been conventionally used, but also in fields where it has not been used due to its high price. It will also be possible to expand demand for

Agent Patent Attorney Akira Hirose

Claims (1)

[Claims] 1. In the production of silica glass, which consists of the steps of hydrolyzing silicon alkoxide to form a sol solution, gelling the sol, drying the gel, and firing, N-methyl- A method for producing silica glass, characterized by adding pyrrolidone and silica fine particles.