JPH01215728A - Production of quartz glass - Google Patents

Abstract

PURPOSE:To obtain a sintered dense cristobalite-type material and produce a quartz glass free from bubbles and OH group from an alkyl silicate using a sol-gel process, by gelling the sol in the presence of a crystallization promoter. CONSTITUTION:An alkyl silicate such as tetraethoxysilane is hydrolyzed with an acidic or basic catalyst to obtain a sol. An acidic or basic catalyst is added to the sol in the presence of 5-90ppm of a solution of a crystallization promoter such as sodium chloride and the mixture is gelled by maintaining at room temperature. The formed gel is heated at 50-80 deg.C and the obtained dry gel is calcined in a furnace at 1,000-1,400 deg.C for 5-20hr to obtain a dense sintered cristobalite. The sintered material is heated in an electric furnace at >=1,800 deg.C in vacuum or in an inert gas atmosphere such as helium to obtain a transparent quartz glass.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing quartz glass, particularly for use in optical instruments such as lenses and prisms, and in the semiconductor industry since it does not substantially contain bubbles and OH groups. The present invention relates to a method for producing quartz glass, which is useful for crucibles, furnace tubes, etc., using a sol-gel method.

(Conventional technology and problems to be solved) The method for manufacturing quartz glass involves dissolving alkyl silicate in an organic solvent such as ethanol.

Silica sol is produced by hydrolysis in the presence of an acidic catalyst such as hydrochloric acid or a basic catalyst such as ammonia.

The method of dehydrating and condensing this to obtain silica gel, and then drying and firing it to obtain quartz glass is known as the so-called sol-gel method.

However, the silica glass obtained by this method tends to contain fine bubbles and has 500 to 1,000 OH groups.
Because it contains OPP, there is a quality problem in that it is not suitable for optical applications where the presence of bubbles is averse, and for semiconductor industry applications that require heat resistance, so special degassing and deOH treatments are performed for this purpose. There is a need.

For this defoaming treatment, a method is known in which dry gel obtained by the sol-gel method is heat treated in a helium gas atmosphere, and for OH removal treatment, this dry gel is heated to 800°C or higher in a chlorine gas atmosphere. A heat treatment method is used, but if you try to perform this degassing treatment and deOH treatment at the same time, complicated and precise process control is required.
Since this OH removal process involves handling chlorine, which is a toxic gas, at high temperatures, it requires strict safety management in terms of work and equipment, and has the disadvantage of requiring ancillary equipment such as waste gas treatment equipment. There is.

On the other hand, for the production of quartz glass that is substantially free of bubbles and OH groups, silicon dioxide powder obtained by oxidizing silicon halides such as silicon tetrachloride is used in the presence of a crystallization accelerator such as an alkali metal salt. A method is known in which a sintered body of cristobalite is produced by heat treatment, and then this is treated at high temperature under reduced pressure to dealkalize and vitrify this sintered body to produce quartz glass (Japanese Patent Application Laid-Open No. 1983-1982). No. 58823, No. 61-58824, No. 61-58832, No. 61-
No. 63537, No. 62-30633, No. 62-306
(Refer to each publication No. 34) According to this method, it is possible to obtain quartz glass that does not contain bubbles and OH groups relatively easily, but in this method, the starting material is silicon dioxide powder, and this silicon dioxide powder is used as a starting material. Since the crystallization promoter is added to the powder by impregnating it with a crystallization promoter solution, the concentration of the crystallization promoter in the silicon dioxide powder varies depending on the particle size of the silicon dioxide powder, and inevitably the cristobalite This cristobalite formation progresses on a particle-by-particle basis, resulting in large grain sizes of the cristobalite obtained, and when the sintered body is cooled, the phase changes from β phase to α phase at 220 to 275°C. There is a disadvantage in that cracks occur due to the strain caused by the volume change during the transition, so the cristobalite obtained by sintering must be kept above its phase transition temperature before being transferred to the next process. There are some drawbacks.

(Structure of the Invention) The present invention relates to a method for producing quartz glass that solves these disadvantages, and this invention involves adding a crystallization promoter to a sol obtained by hydrolyzing an alkyl silicate, which is uniformly dissolved in the sol. is added and gelled, this gel is dried and heat treated to form a dense sintered body of cristobalite, and then this sintered body is heated in a vacuum or under an inert gas atmosphere to form transparent vitrification. That is.

That is, the present inventors have conducted various studies on a method for producing quartz glass that is substantially free of bubbles and OH groups by the sol-gel method. When added to the sol obtained by hydrolysis of silicate, the crystallization accelerator can be added very easily and completely uniformly into the sol, so if the gel is dried and then heat-treated, the crystallization accelerator can be added. A homogeneous and dense listobalite sintered body in which cristobalite is uniformly dispersed at the molecular level can be obtained, and since this cristobalite is obtained as a very fine and dense sintered body, the transition from β phase to α phase occurs. It does not break due to cracking during phase transition, and is stable even when cooled to room temperature. Also, when this cristobalite sintered body is heated in a vacuum or in an inert gas atmosphere, crystallization, which is an impurity in silica glass, occurs. It has been found that the accelerator can be completely removed by this heat treatment, and that no bubbles are generated and OH groups can also be eliminated.

Each step of the present invention will be explained in more detail below.

In the method of the present invention, a sol is first prepared by hydrolyzing an alkyl silicate, which can be carried out by any known method. 1 mole of silicate ester such as silane or tetrabutoxysilane, 4 to 20 moles of water, and an organic solvent such as methanol.

Alcohol such as ethanol may be added, and 1/100 to 1 mole of hydrochloric acid, nitric acid, etc. as an acidic catalyst, or aqueous ammonia, formamide, etc. as a basic catalyst may be added thereto, and hydrolysis may be carried out at room temperature.

In the method of the present invention, a crystallization promoter is added to the sol obtained by hydrolysis of alkyl silicate. good. Examples of crystallization accelerators include compounds of metals with high vapor pressure such as alkali metals (1), such as their chlorides, hydroxides, oxides, alkoxides, etc., which are soluble in organic solvents such as the alkyl silicates, water, and alcohols mentioned above. Therefore, sodium chloride, sodium hydroxide, sodium hydrogen carbonate, etc. are exemplified, and these may be used as an aqueous solution of about 1 to 10%.

Further, the amount of the crystallization promoter added is, for example, N
100-2. Although it is necessary to add OOOppm, in the method of the present invention, it is uniformly added to the alkyl silicate sol in the form of a solution, so a smaller amount of 5 to 90 ppm, for example 20 ppm, may be used, thereby reducing the hydrolysis of the alkyl silicate. The sol obtained is said to contain a necessary and sufficient amount of crystallization promoter.

In the method of the present invention, the sol containing the crystallization accelerator thus obtained is gelled and then sintered to form cristobalite. It is sufficient to add an acidic catalyst such as or a basic catalyst such as aqueous ammonia or formamide.The amount of these catalysts may be 1 to 100% based on the sol.For gelation, leave it at room temperature after adding this catalyst. This can be easily done by The gel thus obtained is made into a dry gel by heating to 50 to 80°C, and then sintered to form cristobalite. It is sufficient to heat the gel to ~1.400° C. for 5 to 20 hours, and the dry gel becomes dense through this sintering to form a listoparite sintered body. Furthermore, since this sintered body is very fine and dense, even when it passes through 220 to 275 degrees Celsius, which is the phase transition point from β phase to α phase during cooling, the strain caused by the volume change is small. Therefore, since cracks do not occur, this product can be cooled to room temperature by cooling.

In the method of the present invention, this cristobalite sintered body is melted to produce quartz glass. All you have to do is make it In this step, it is necessary to volatilize and remove the crystallization promoter and other impurities added in the above-described step, but this may be carried out in a vacuum or in an inert gas atmosphere such as helium. When processed in this way, the cristobalite sintered body is melted and vitrified to produce transparent quartz glass, but since the crystallization accelerator and other impurities have already been removed from the sintered body, the purity of this quartz glass cannot be improved. This also means that there are no air bubbles and OH groups have been removed by the melting, so there are virtually no air bubbles and O2 groups.
It is considered to be transparent, high-purity quartz glass that does not contain H groups.

As described above, in the method of the present invention, a crystallization accelerator is added to the sol in the sol-gel method, so it is easy to convert the gel into cristobalite, and as a result, it is easy to produce synthetic quartz that does not substantially contain bubbles or OH groups. This gives the advantage of being able to obtain

Next, examples of the present invention will be given.

Example 1 0.1 g of a 2.5% sodium monochloride aqueous solution was added to a silica sol obtained by adding 5 mol of water and 0.005 mol of hydrochloric acid to 0.5 mol of ethyl silicate and hydrolyzing it. .2
Silica gel was prepared by adding N aqueous ammonia and adjusting the pH to 3.5.

Next, this silica gel was dried at 60°C for 10 days, and a dried gel with a Na concentration of 30PpH was obtained.The temperature of this was raised stepwise to 1,350"C in the air, and the gel was dried at 1,350"C. After sintering for 10 hours, 200
When it was cooled to room temperature at a rate of .degree. C./min, no cracks were generated, and a dense sintered body of listalite was obtained.

Next, this sintered body was inserted into a vacuum heating furnace and heated to 1,800°C for 30 minutes under a vacuum of 10-4 Torr.
A transparent quartz glass body of φX5tm+m was obtained, and there was no strain point measured by a polarization strain meter, and its OH group concentration was examined by infrared analysis and found to be OI (concentration is Oppm, measured by atomic absorption spectrometry). The residual Na concentration was 0.5pp.
+s or less.

Comparative Example 1 A dry gel was prepared in the same manner as in the above example except that sodium chloride as a crystallization accelerator was not added.
When the temperature was raised stepwise to 1,350°C for 10 hours and then cooled to room temperature at a rate of 200°C/min, a transparent quartz glass body was obtained.
Contains many microscopic bubbles of about m size, and this bubble is about 1 of them.
0% had distortion as seen with a polarization strain meter, and the OH concentration as measured by infrared spectroscopy was 570 pp.

Comparative Example 2 0.005 mol of hydrochloric acid was added to 0.5 mol of ethyl L1 acid and 5 mol of water and hydrolyzed at room temperature to produce silica sol, which was adjusted to pH 3.5 to form silica gel and heated at 60°C.
After drying for 20 days to produce a dry gel, this was ground in a ball mill to produce 30 g of fine quartz powder with a particle size of 20 to 100 μm.

Next, this quartz powder was impregnated with a 5% sodium chloride solution and dried, and the Na concentration was 1.
Dry quartz powder with a concentration of 200 ppm was obtained, so it was pressed into a tablet shape in a mold, heated stepwise to 1,350°C, heated at 1,350°C for 10 hours, and then
When the cristobalite sintered body was cooled to room temperature at a rate of 00° C./min, it cracked.

Claims (1)

[Claims] 1. A sol obtained by hydrolyzing an alkyl silicate is gelled in the presence of a crystallization accelerator that is uniformly dissolved in the sol, and this gel is dried and then heat-treated to form a dense crystal of cristobalite. A method for producing quartz glass, which comprises forming a sintered body, and then heating the sintered body in a vacuum or an inert gas atmosphere to turn it into transparent glass. 2. The method for producing quartz glass according to claim 1, wherein the crystallization promoter is selected from alkali metal compounds.