JPH04238824A - Manufacturing method of high viscosity synthetic quartz glass for optical use - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is directed to a method for producing high-viscosity synthetic quartz glass for optical use, and in particular, since a completely bubble-free and highly viscous synthetic quartz glass can be obtained, it is useful for optical use such as TFT substrates. The present invention relates to a method for producing high-viscosity synthetic silica glass for optical use.

0002

[Prior Art] A plasma method is known for producing anhydrous and bubble-free synthetic quartz glass, in which silicon tetrachloride gas is introduced into oxygen plasma to cause hydrolysis and sintering. It is disadvantageous in that it is very expensive, contains Cl2, and has severe striae. Therefore, regarding this, silicon tetrachloride is placed in an oxyhydrogen flame, and the silica fine powder generated by the flame hydrolysis is deposited on a carrier to create a porous glass base material, which is then sintered to create a synthetic silica glass. The mainstream method is called soot, which requires high-temperature Cl2 treatment or sintering the soot in a vacuum to make it completely anhydrous. It has the disadvantage of being difficult to mass produce due to its small size and being expensive.
Furthermore, external heating causes a difference in OH group content between the outside and the center, resulting in a non-uniform glass, which has the disadvantage of producing weak striae.

Regarding the production of this synthetic quartz glass, Zarzycki et al. hot-pressed a gel obtained by hydrolyzing alkoxide under vacuum to form SiO2, La
2O3-SiO2, B2O3-SiO2 type glasses have been obtained (J. Mat. Sci., 13 (197
8), 2605-18), but since the raw material is gel and contains a large amount of water, the gel must be heated under pressure to prevent foaming or clouding at high temperatures and vacuum. A disadvantage is that a flash pressing method must be used, in which heating is continued and heating is stopped when a predetermined temperature is reached.

0004

[Problems to be Solved by the Invention] For this reason, the present inventors have conducted research on a method for easily and inexpensively obtaining high-viscosity synthetic quartz glass, which can be obtained by hydrolyzing methyl silicate with aqueous ammonia. We discovered that colloidal silica easily becomes anhydrous when heated, and that synthetic silica glass with high viscosity can be obtained by vacuum sintering it (see Japanese Patent Application No. 1-139619). It has not yet been possible to make quartz glass completely bubble-free and use it as an optical substrate material. The reason for this is closely related to the bulk specific gravity at the time of preparation; if the bulk specific gravity does not increase, the gaps between the particles will become large and bubbles will eventually remain.

[0005]

[Means for Solving the Problems] The present invention relates to a method for producing optical high-viscosity synthetic quartz glass that solves the above-mentioned problems. After decarburizing the silica powder, it is sieved to a particle size of 150 to 200 mesh, and this is
) Packed in a carbon mold, applying pressure of 1 kg/cm2 or more from above and below, 2) 1,200 to 1,400 °C, H
3) After the pressure is reduced, the temperature is raised to 1,500 to 1,800 °C at 10-1 Torr or higher, and 4) When the sintered body reaches the theoretical density, the pressure and vacuum are released. 5) heated to 1,800° C. or higher at normal pressure.

That is, the present inventors have conducted various studies on the manufacturing method of high-viscosity synthetic quartz glass, which is useful as an optical substrate because it is completely bubble-free and has no bright spots. If we identify the alkoxysilane as methyl silicate and sufficiently refine it and then hydrolyze it in the presence of aqueous ammonia, we can obtain silica with a three-dimensional matrix structure with large particle and pore sizes. It was sieved to ~200 mesh, then pressed in a carbon mold, and sieved to 1,200 mesh in He.
After holding at ~1,400℃, 1,
By sintering at 500 to 1,700°C, releasing the pressing pressure and reduced pressure, and finally heating it to normal pressure at 1,800°C, completely bubble-free, high-viscosity synthetic quartz glass can be easily and inexpensively produced. The present invention was completed by conducting research on the treatment conditions in each step. This will be explained in further detail below.

[0007]

[Operation] The present invention relates to a method for producing optical high-viscosity synthetic silica glass, which involves hydrolyzing methyl silicate in the presence of ammonia to produce silica particles, and then decarburizing the particles. It is sintered and heat treated to form quartz glass.

The starting material used in the method of the present invention is highly reactive, and in the presence of ammonia, it easily forms aggregated spherical silica with a particle size of 50 to 150 μm without the risk of gelation. Methyl silicate is selected because it easily becomes anhydrous when heated, but since the desired synthetic quartz glass is to be of high purity, methyl silicate is used in advance by distillation etc. It must be supplied as a sufficiently purified product.

This methyl silicate is made into silica sol by hydrolysis, but if this hydrolysis is carried out in the presence of a known acid catalyst such as hydrochloric acid, the resulting silica will have small particles, so ammonia It must be carried out in the presence of a catalyst. When this methyl silicate is hydrolyzed in the presence of an ammonia catalyst, the resulting silica aggregates into large particles with a particle size of 50 to 150 nm.

[0010] This hydrolysis reaction may be carried out at 40 to 50°C, and the silica sol thus obtained may be separated into silica by solid-liquid separation using a filter press, for example. Since organic matter remains during hydrolysis, it is necessary to decarburize it, so it is necessary to heat it in air at 800 to 1,200°C. If this decarburized colloidal silica has a wide viscosity distribution, sintering will proceed gradually at lower temperatures in the next heating step, making it easier for bubbles to be contained inside, so it is necessary to sieve it to a predetermined particle size. If it is larger than 150 mesh, the gaps between the particles will become large and fine bubbles will remain even after sintering, and if it is smaller than 200 mesh, the shrinkage during sintering will increase and cracks will occur. bubbles will be generated, so this is 150 to 200
It is best to pass through a mesh sieve.

The colloidal silica sieved in this way is then sintered, but this must be done in a carbon mold because it is in a high-temperature vacuum.
The bong mold may be made of isotropic, high-purity material. The colloidal silica packed in this carbon mold is pressurized to have a bulk specific gravity of 0.8 to 1.0, but if this pressure is less than 1 kg/cm2, the bulk specific gravity will not rise sufficiently and the bulk specific gravity will be 10 to 1.0. Since 20 μm bubbles will ultimately remain, this is more than 1 kg/cm2, e.g. ~ kg/cm2.
It may be set to cm2.

Next, after this pressure treatment, the inside of this carbon mold is initially maintained at 1,200 to 1,400° C. in a He atmosphere. This is necessary because in a He atmosphere, heating is transmitted to the entire sintered body and it contracts uniformly, so there is no unmelted portion. After this, it is further held in a vacuum and sintered at a high temperature under this reduced pressure, the degree of reduced pressure being 10-1 torrents.
If the temperature is less than 10 Torr, the resulting synthetic silica glass becomes a bright spot even if no bubbles remain, making it unsuitable for optical applications. It may be set to 10-1 to 10-3 Torr. In addition, if the sintering temperature is lower than 1,500°C, sufficient sintering will not occur and the appearance will become opaque due to the heating in the next step, while if the temperature is higher than 1,800°C, the silica particles will not be contained. This should be in the range of 1,500 to 1,800°C as the bubbles grow and become larger until they last.

[0013] Note that this temperature of 1,500 to 1,700°C
If the temperature is gradually increased, the pores of the silica will begin to form from the outside and bubbles will be trapped inside, so this should be done at a rate of 10°C/min or more, preferably 40°C/min or more. That's good. For example, this is done by heating to 1,200 to 1,400 °C, at which point no pore closure occurs, and then
, 500 to 1,800°C.

Next, the theoretical density of quartz glass is 2.2.
When the temperature reaches 0.03 °C, the pressure and vacuum are released, the sample is taken out from the carbon mold, and it is heated to 1,800 °C or higher under normal pressure to completely vitrify it. This heating temperature is 1.8
If the temperature is higher than 00°C, the viscosity of the quartz glass becomes low, and the bubbles generated during the previous stage of vacuum sintering tend to collapse due to the pressure difference.
This is preferably in the range of 1,800 to 2,000°C, since SiO vapor and Si metal will be generated vigorously if the temperature is above 2,000°C, although this provides the advantage of no visible bubbles. It is good to do. Note that this heating may be carried out under pressure of 1 to 10 atmospheres, which has the advantage that the bubbles are further crushed and become completely invisible to the naked eye.

[0015]

[Example] Next, an example of the present invention will be given. Example 26.5 ml of methyl silicate was added to a 5 liter continuous flask.
liter/hour and 20.5% ammonia water/EL gray
When colloidal silica [trade name manufactured by Omori Kako Co., Ltd.] was simultaneously dropped at a rate of 25.8 liters/hour at a rate of 25.8 liters/hour, agglomerated colloidal silica was obtained at a rate of 10 kg/hour. Once left alone, polypropylene 1,000
Solid-liquid separation was performed using a centrifugal dehydrator equipped with a 0# filter cloth, and then washed five times with ultrapure water. This agglomerated colloidal silica was packed into a quartz glass furnace core tube, and heated from room temperature to
,000°C over 10 hours and held for 2 hours to decarburize.

[0016] Next, this powder was poured into a polypropylene 1
After sifting through a 50-mesh screen, this 2 kg was sieved with an outer diameter of 160 mmφ, an inner diameter of 130 mmφ, and a height of 400 mm.
The mixture was filled into a carbon mold and placed in a vacuum hot press furnace. The inside of this furnace was made into a He atmosphere, and the temperature was raised to 1,300 °C for 1 hour.
After raising the temperature to and maintaining ZH, use a vacuum pump to
The pressure was reduced to 0-2 Torr and a load of 500 kg was applied. When the temperature was raised to 1,650 °C for 30 minutes and held for 10 minutes, the specific gravity of the glass became 2.203, so the load and vacuum were released, and the temperature was raised to 1,850 °C for 30 minutes.
When I raised the temperature for 30 minutes, held it for 30 minutes, left it to cool, and took out the product, the dimensions were 130mm φ x 60m thick.
Synthetic quartz glass was obtained as calculated by m, and it was blue (excitation wavelength 245 nm), completely bubble-free, and no striae was observed, and the transition temperature was 1,070 °C at the strain point and 1,17 at the annealing point.
It exhibited physical properties of 5° C. and a maximum operating temperature of 1,100° C., and the impurity analysis values were as shown in Table 1, indicating that it was a high purity product.

[0017]

[Table 1]

[0018]

Effects of the Invention The present invention relates to a method for manufacturing high-viscosity synthetic quartz glass for optical use, and as described above, this involves decarburizing colloidal silica powder obtained by hydrolyzing methyl silicate in aqueous ammonia. , particle size 150-200
Sieve through a mesh, pack it into a carbon mold,
Apply pressure of kg/cm2 or more from above and below, 2)1,
200 to 1,400 °C, maintained in He atmosphere, 3)
1,500 to 1 at a reduced pressure of 10-1 Torr or higher
4) When the sintered body reaches the theoretical density, pressurization and depressurization are released and the sintered body is taken out.
5) It is characterized by heating to 1,800 °C or higher at normal pressure, and by this method, completely bubble-free and highly viscous synthetic quartz glass can be obtained easily and inexpensively. The advantage is that it is possible to easily obtain optical high-viscosity synthetic quartz glass that is useful for optical applications such as TFT substrates.

Claims (1)

[Claims] Claim 1: Colloidal silica powder obtained by hydrolyzing methyl silicate in aqueous ammonia is decarburized and then sieved to a particle size of 150 to 200 mesh. The above pressure is applied from above and below, 2) the temperature is maintained at 1,200 to 1,400 °C in a He atmosphere, and 3) the temperature is raised to 1,500 to 1,800 °C at a degree of vacuum of 10-1 Torr or more. 4) When the sintered body reaches the theoretical density, the pressure and vacuum are released to form a sintered body, and 5) High viscosity synthesis for optical use is characterized by heating to 1,800 °C or higher at normal pressure. Method for manufacturing quartz glass.