JPH0517172B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing quartz glass, and in particular to a method for producing quartz glass, which has high purity and heat resistance equivalent to natural quartz glass, and is therefore useful as a heat treatment jig in the semiconductor industry. This relates to a manufacturing method. (Prior art) For the production of high-purity quartz glass, natural quartz is purified by washing and sorting, or the obtained natural quartz glass is electrolyzed (International Publication Patent W086/02919)
), or by hydrolyzing or thermally decomposing a silicon compound such as silicon tetrachloride in an oxyhydrogen flame or plasma flame to produce silica, which is then melted into glass. However, products made from natural quartz have poor purity for use in the semiconductor industry, where the degree of integration is increasing, and synthetic quartz glass made from silicon compounds contains a large amount of OH groups. However, they have the disadvantage of having problems with dimensional stability at high temperatures and being very expensive. Therefore, the so-called sol-gel method is used to produce high-purity silica glass, which involves creating a gel from an alcoholic solution of a metal alkoxide or an inorganic silicate such as water glass, and then firing it to create transparent silica glass. According to this proposal, the amount of impurities is 1/10 to 1/100 compared to natural quartz glass, and the homogeneity of the glass is good, and the energy cost is significantly lower than that of synthetic quartz glass, making it cheaper. Although it provides the convenience of being able to obtain quartz glass, it has the disadvantage that the quartz glass will crack if the temperature is not precisely controlled during the manufacturing process, and it will foam if used above the firing temperature. There are several points that make this measure problematic. (Structure of the Invention) The present invention relates to a method for producing quartz glass that solves these disadvantages, and this involves heating amorphous silicon dioxide produced by a sol-gel method using a metal alkoxide or an inorganic silicate as a raw material. The process is characterized in that it is converted into β-cristobalite, which is then cooled to transform into α-cristobalite powder, and then melted and vitrified to form quartz glass. That is, as a result of various studies conducted by the present inventors on methods for solving the disadvantages of the known sol-gel method, the present inventors have found that, in accordance with the conventionally known sol-gel method, an alcohol solution of a metal alkoxide or an aqueous solution of an inorganic silicate such as water glass can be After producing amorphous silicon dioxide, it is heated to form β-cristobalite, and then cooled to form α-cristobalite powder. If this is melted and vitrified, it becomes high-purity, heat-resistant quartz. It has been discovered that the glass can be easily obtained without the need for post-treatments such as electrolysis, and the β
−Heating method for obtaining cristobalite, α−
The present invention was completed by conducting research on methods for purifying cristobalite powder. In the method of the present invention, first, amorphous silicon dioxide is produced from a metal alkoxide or an inorganic silicate, and this may be carried out by a known sol-gel method. In this sol-gel method, a mixture of water and ethyl alcohol of a metal alkoxide such as tetraethoxysilane [Si(OC ₂ H ₅ ) ₄ ] or an aqueous solution of an inorganic silicate such as water glass is added to agar under heating. After forming a sol into a sol, this is heated to drive out alcohol and water to obtain silicon dioxide as a dry gel. According to this method, silicon dioxide is obtained in an amorphous form. In the method of the present invention, the amorphous silicon dioxide obtained by this known sol-gel method is then heat-treated to obtain β-cristobalite.
- It is necessary to carry out the process at a temperature below 1723°C, which is the melting point of cristobalite, but it is particularly preferable to carry out the process at 1630°C, where the rate of transformation to β-cristobalite is maximum. For example, when this amorphous silicon dioxide is heated at 1630°C for 30 hours, it completely turns into β-cristobalite. Furthermore, when this transition to β-cristobalite is carried out in the presence of an alkaline component such as a hydroxide of an alkali metal such as sodium or potassium, the transition to β-cristobalite is 10 to 100 times greater than when no alkali component is added. Transfer speed is 1000
Achieved at low temperatures such as ~1200℃. for example
By heating to 1200°C in the presence of NaOH, amorphous silicon dioxide can be converted into β-cristobalite in 3 hours. In the method of the present invention, the β-cristobalite thus obtained is then converted into α-cristobalite powder, but this cristobalite exists as β-cristobalite at high temperatures but becomes α-cristobalite at low temperatures.
-Transition to cristobalite is achieved by simply cooling β-cristobalite obtained by heating to room temperature as described above. As a result of this cooling, a rapid volumetric contraction occurs and cracks occur when the material transforms into α-cristobalite, so that powdered α-cristobalite can be easily obtained.
In this case, if the amorphous silicon dioxide used as the starting material is below a certain particle size, cracks will not occur and the particles will simply shrink and will not be pulverized into finer particles. Therefore, the α-cristobalite powder obtained has an appropriate particle size distribution and does not contain fine powders which are inconvenient for later melting and vitrification. Specifically, it is preferable to use 325 meshes. Due to the generation of cracks caused by the above-mentioned transition from β-cristobalite to α-cristobalite, the α-cristobalite powder normally obtained has a particle size of 9 mesh or less.
All of these materials can be made into quartz glass by vacuum melting or oxyhydrogen flame melting, but in order to obtain high-purity quartz glass, it is not preferable that the material is too coarse.
That is, if alkali is added to convert cristobalite and then purification is performed, the purification effect will be insufficient for coarse particles. Specifically, the purification effect is good when the mesh size is 32 meshes or less. In short, α-cristobalite powder of 32 mesh or lower and 325 mesh is most preferable for producing high-purity quartz glass that does not contain bubbles. The production of quartz glass according to the method of the present invention is carried out by melting and vitrifying the α-cristobalite powder obtained in this way. Since this alkaline component remains, it is best to purify this material in advance. Therefore, this material must be treated with hydrofluoric acid, hydrochloric acid, nitric acid, etc. before melting and vitrification. Clean the surface with acid or heat it to about 1500℃ in air or 1300℃ in a halogen gas atmosphere.
It is best to heat-treat to a certain degree. The thus purified α-cristobalite powder can be melted and vitrified by heating the α-cristobalite powder to a temperature above its melting point of 1723°C, but this can be done by oxyhydrogen flame melting or electric melting. do it. Note that if the amorphous silicon dioxide obtained above is directly melted in an oxyhydrogen flame without converting it into cristobalite, since the particles are amorphous, only quartz glass containing a large amount of occluded gas and having bubbles can be obtained. Particularly in the case of electric melting, generation of bubbles due to occluded gas is a problem. In the case of oxyhydrogen flame melting, it is possible to obtain quartz glass with fewer bubbles by slowing down the deposition rate (growth rate), but since the number of OH groups introduced by the flame increases, the resulting quartz glass does not have sufficient heat resistance. It does not become something that has. However, the method of the present invention has the industrial advantage of being able to easily obtain silica glass with a low OH group content and with few bubbles. As described above, the method of the present invention involves producing amorphous silicon dioxide from a metal alkoxide or an inorganic silicate such as water glass by a sol-gel method, then converting this into β-cristobalite, and then converting it into α-cristobalite powder. Silica glass is obtained by melting and vitrifying it, but prior to melting, amorphous silicon dioxide is turned into crystals with an almost stoichiometric composition free of crystal defects, occluded gas, and water, so it is highly efficient. It is possible to obtain bubble-free quartz glass with a high viscosity. At the same time, since crystals with a stoichiometric composition are difficult to contain impurities, a purifying effect can be expected by converting them to cristobalite. Next, examples of the present invention will be given. Example 1 1 mole of tetraethoxysilane (Si(OC ₂ H ₅ ) ₄ ), 3 moles of 0.01 N aqueous ammonia and 4 moles of methanol were mixed and stirred to prepare a sol solution, and this sol was made into plastic. After pouring it into a beaker and leaving it in a constant temperature bath at 60℃ for 100 hours, the temperature was raised to 1100℃ at a temperature increase rate of 200℃/h in an electric furnace and kept at the same temperature for 2 hours. Amorphous silicon dioxide was obtained. This was ground in a ball mill made of synthetic quartz glass, and then classified with a nylon sieve to obtain a powder (sample 1) with a size of 325 mesh and below 32 mesh. Next, this powder was immersed in a 0.5% NaOH aqueous solution, filtered, and dried at 100°C to remove trace amounts of Na.
A powder in which remains uniformly was obtained. This powder was heated in an electric furnace at 1200°C for 10 hours to form β-cristobalite, and then cooled to room temperature to form α-cristobalite powder. This was classified to obtain a powder (sample 2) of 325 meshes and 32 meshes below. This powder was immersed and cleaned in 0.1N _HNO3 , then heated at 1150°C for 10 hours in a mixed gas flow of chlorine gas and hydrogen chloride in a molar ratio of 1:2 to perform dealkalization, and then classified again. A powder of 325 meshes and 32 meshes below (sample 3) was obtained. Next, Samples 1 to 3 obtained in this manner, and for comparison, natural quartz powder (sample 4) and synthetic quartz glass powder (sample 5) with particle sizes of 32 mesh and 325 mesh were heated to 1750°C in an electric furnace in a vacuum atmosphere. The glass was melted for 1 hour to form a glass lump, and the bubble state and Na content of this glass were examined, and the results shown in Table 1 were obtained.

[Table] Analyzed.
In addition, samples 1 to 5 were melted and vitrified using an oxyhydrogen flame melting method, and the OH group content in the bubble state and 1150℃
When the viscosity was examined, the results shown in Table 2 were obtained.

[Table] Example 2 Composition components are Na ₂ O 10% by weight, SiO ₂ 29% by weight,
Prepare 500 g of an aqueous solution with a SiO ₂ content of 15% by diluting JIS No. 3 water glass containing 61% by weight of H ₂ O with distilled water, and add 1N hydrochloric acid with stirring to gel it. After drying this gel at 100°C for 50 hours, the temperature was raised to 1000°C at a heating rate of 200°C/h in an electric furnace under an inert gas atmosphere, and heated at the same temperature for 3 hours to form powdered amorphous dioxide. Silicon was prepared, and this powder was crushed and classified in the same manner as Sample 1 to obtain a powder (Sample 6) with a size above 325 meshes and below 32 meshes. This powder was converted to β-cristobalite in the same manner as Sample 2, and then classified to produce α-cristobalite powder (Sample 7) with 325 meshes and 32 meshes lower, and this α-cristobalite powder was dealkalized in the same manner as Sample 3. , and classified to obtain a powder (sample 8) with a size of 325 meshes and 32 meshes below. In addition, this powder was further crushed and classified to produce a fine powder (sample 9) with a size of 325 mesh or less. A powder (sample 10) similar to sample 8 but with a classification range of 32 meshes above and 9 meshes below was prepared. Next, sample 4 described above was heated at 1600°C for 30 hours and cooled without adding NaOH aqueous solution, and α
- It was converted into cristobalite, which is a porous and weak sintered body, so it was lightly crushed to obtain a powder (sample 11) of 325 mesh and 32 mesh. Next, Samples 6 to 11 obtained in this way were melted at 1750℃ for 1 hour in an electric furnace in a vacuum atmosphere to make a glass lump, and the state of the bubbles and Na content of this glass were examined. The results shown in were obtained.

[Table] In addition, Samples 6 to 11 were melted in an oxyhydrogen flame and vitrified, and the foam state, OH group content, and viscosity at 1150°C were examined, and the results are shown in Table 4. The results shown in were obtained.

[Table] As explained in detail above, first, as shown in Samples 3, 8, 10 and 11 in Tables 1 and 3, bubbles were removed from high-purity α-cristobalite powder by vacuum melting. It is clear that good quartz glass can be obtained. Second, as shown in Samples 3, 8, 10 and 11 in Tables 2 and 4, silica glass with no bubbles and good heat resistance can be obtained from α-cristobalite powder. If there is a residual alkali as shown in Samples 2 and 7, or if there is a fine powder as in Sample 9, the growth rate will be slower, but almost the same good quartz glass will be obtained. Based on the above comprehensive evaluation, quartz glass with excellent purity, foam, and heat resistance cannot be obtained from conventional natural quartz glass or synthetic quartz glass, but can only be obtained from optimal samples 3 and 8 of the present invention. it is obvious.

Claims (1)

[Claims] 1. Amorphous silicon dioxide produced by a sol-gel method using a metal alkoxide or inorganic silicate as a raw material is heated to form β-cristobalite, which is then cooled and transformed into α-cristobalite powder. A method for producing quartz glass, which comprises: 2. The method for producing quartz glass according to claim 1, wherein amorphous silicon dioxide is converted into β-cristobalite by heating in the presence of an alkali component. 3. The method for producing quartz glass according to claim 1, wherein the α-cristobalite powder is less than 9 meshes. 4 α-cristobalite powder is below 32 meshes,
325 mesh first claim
The method for producing quartz glass described in Section 1. 5. Claim 1 in which the molten vitrification is performed after purifying α-cristobalite powder
The method for producing quartz glass described in Section 1. 6. The method for producing quartz glass according to claim 5, wherein the purification treatment of the α-cristobalite powder is a heat treatment, a heat treatment in the presence of a halogen component, or an acid washing treatment, or a combination thereof.