JPH11349337A - Production of synthetic silica glass powder and production of silica glass molded form - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain synthetic silica glass powder by hydrolyzing an alkoxysilane wherein the system is cooled to enough low temperatures to prevent evaporated components from condensation/gelation, and by baking the resultant gel so as to suppress the development of black foreign matter and reduce bubbling in the molding process for the silica glass powder in a molten state. SOLUTION: This synthetic silica glass powder is obtained by the following process: a high-purity alkoxysilane such as tetramethoxysilane is hydrolyzed with about 1-10 equivalent times of pure water to form a sol which is then gelatinized at normal or higher temperatures; the resulting silica gel is ground and dried at about 50-200 deg.C into powder about 5-30 wt.% in liquid content and about 10-1,000 μm in particle size; the dried silica gel powder is subsequently heat-treated pref. at 300-500 deg.C or so and then baked at about 1,000-1,300 deg.C; wherein during the hydrolysis process, spots where evaporated components may condensate are cooled to <=10 deg.C (pref. <=5 deg.C or so); thereby affording the aimed synthetic silica glass powder free from any black foreign matter. The other objective silica glass molded form with extremely low bubble generation is obtained by molding the above synthetic silica glass powder in a molten state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to the field of semiconductor manufacturing and optical fields such as optical fiber.
The present invention relates to a synthetic quartz glass powder capable of obtaining an extremely small amount of bubbles in the product, and a quartz glass product obtained by using the same.

[0002]

2. Description of the Related Art In recent years, glass products used in the optical communication field, the semiconductor industry, and the like have undergone very strict control over their quality. As a method for producing such high-purity glass, (1) a method of using purified natural quartz, (2) hydrolysis of a gaseous silicon compound such as silicon tetrachloride in an oxyhydrogen flame, Synthetic quartz powder obtained by calcining silica gel powder obtained by vapor phase method of fusing and generating fumes generated by thermal decomposition, and (3) hydrolysis in liquid phase such as silicon alkoxide, gelation, etc. The method used and the like are known.

[0003]

However, the method (1) has a limit in reducing the content of trace impurities, and the method (2) has a problem that the production cost is extremely high. In the method (3) using silica gel, particularly the method using silica gel powder using silicon alkoxide as a raw material, a synthetic quartz glass having a low trace impurity content can be obtained at lower cost than the method (2).

Here, the quality required of the quartz glass powder is required not only to have a low level of metal impurities but also to generate less bubbles in a quartz glass product obtained by melting and molding the quartz glass powder. I have. When bubbles are generated in a quartz glass product, when used at a high temperature in a member for semiconductor manufacturing, it causes a reduction in strength.In addition, impurities accumulate on the surface of the bubble, and in a crucible for pulling a silicon single crystal, the yield decreases. It becomes a factor. In optical applications,
It becomes a light scattering starting point, causing a decrease in transmittance, and in the production of an optical fiber, may cause a disconnection at the time of stretching.

[0005] In particular, when quartz glass powder obtained by a sol-gel method using silicon alkoxide as a raw material is used, when the sol-gel reaction is carried out in a batch system, if the number of reaction batches increases, the inner wall portion of the reaction vessel or a stirring blade is required. Etc., scales are generated and peel off when the thickness of the scale grows to some extent,
Mix in products. This scale is originally silica gel produced by a hydrolysis reaction, but as a result of studies by the present inventors, this scale has a denser structure than a normal gel, It is found that the carbon component derived from the alkoxy group or the by-product alcohol is difficult to be desorbed at the time of being formed, so that it is liable to become a black foreign substance in the quartz glass powder, which becomes CO or CO ₂ gas at the time of melting and causes bubbles. (JP-A-6-34041)
No. 1, JP-A-8-188411). For this reason, a method has been proposed in which the adhered scale is periodically dissolved and removed with an aqueous solution of caustic soda to stably obtain a synthetic quartz glass powder with less foaming during melt molding (Japanese Patent Laid-Open No. 6-340411).

However, in the method based on the treatment with caustic soda, the caustic soda used for dissolution and removal may remain in the dead space of the reactor depending on the structure of the reactor, and a considerable amount of batch may be obtained after the start of the hydrolysis reaction. There is a problem that a number of products may be contaminated with Na and productivity is reduced. As a method of making it difficult to generate black foreign matter in quartz glass powder, there is known a method of treating a gel obtained by a hydrolysis reaction with superheated steam at 150 ° C. or higher (Japanese Patent Publication No. 5-4925). This method can be expected to be effective for normal gel particles,
The inventors of the present invention have found out that, as for the above-described scales whose properties have been remarkably changed (densification has progressed), even if steam treatment is performed, there is almost no effect.

[0007] As a solution to this problem, a part of the reaction vessel in which the alkoxysilane is hydrolyzed, which is in contact with the hydrolyzate, is periodically treated with hot water of 70 ° C or higher, so that the inner wall of the reactor is treated. It has been found by the present inventors that the formed scale can be modified so as not to be blackened at the time of firing (Japanese Patent Application No. 9-120750). According to this method, product loss such as when washing with alkali is obtained. Not suitable. However, in this method, although an extremely high reforming effect is obtained in a region in contact with hot water, usually, the reactor is in contact with hot water such as a reaction vessel ceiling, a raw material liquid introduction section, and a gas discharge section. There is a non-liquid region, and the scale formed in the portion not in contact with the hot water cannot be expected to have a reforming effect, and cannot be said to be perfect. Especially,
According to the study of the present inventors, in the area not in contact with hot water, the inside surface of the vent line pipe at the time of reaction and the inner surface of the reduced pressure suction pipe, the formation of black foreign matter at the time of firing, extremely bad scale occurs. found.

[0008]

The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, the vent line of the hydrolysis reactor and the reduced pressure suction pipe were cooled to carry out the hydrolysis during the reaction. By preventing condensation and gelation of liquid volatile components,
The present inventors have found that it is possible to obtain a synthetic quartz glass powder in which the generation of black foreign substances is suppressed to an extremely low level, and have reached the present invention. That is, the present invention is characterized in that, in firing a gel obtained by hydrolyzing alkoxysilane to obtain a synthetic quartz glass powder, in a hydrolysis step, a portion where an evaporating component condenses is cooled to 10 ° C. or less. And a method for producing a quartz glass molded body characterized in that the synthetic quartz glass powder thus obtained is melt-molded.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
Production of silica gel powder by hydrolysis of alkoxysilane is performed by reacting alkoxysilane with water according to a method known as a so-called sol-gel method. As the alkoxysilane used as a raw material, a C1-4 lower alkoxysilane such as tetramethoxysilane or tetraethoxysilane or an oligomer that is a low condensate thereof is easily hydrolyzed, and the carbon residue in the silica gel is small. Preferred from the point.

The amount of water used is usually selected from the range of 1 to 10 equivalents of the alkoxy group in the alkoxysilane. At this time, if necessary, an organic solvent such as an alcohol or an ether which is compatible with water may be mixed and used. Representative examples of the alcohol used include lower aliphatic alcohols such as methanol and ethanol. Here, in the present invention, with respect to organic solvents such as alkoxysilane, water, and alcohol introduced into the reaction system, it is preferable to remove foreign substances through a filter before being introduced into the reaction system in advance. Thus, the synthetic quartz glass powder finally obtained by passing through a filter is prevented from being devitrified at the time of melt molding, and a molded article having particularly excellent transparency can be obtained. Although the reason is not clear, it is considered that the particulate foreign matter that can be removed by the filter plays some role in the mechanism of devitrification. The hydrolysis reaction may be carried out in the presence of an acid such as hydrochloric acid or acetic acid or an alkali such as ammonia as a catalyst. It is needless to say that all substances to be introduced into the reaction system, such as water and a catalyst, used here have high purity.

[0011] When the hydrolysis reaction step is carried out in batch, by repeating the number of reaction batches, scale is deposited on the inner wall portion of the reactor and the scale peels off in the product.
It has been found that they are mixed and cause black foreign matter during firing (JP-A-6-340411, JP-A-8-34011).
188411). Therefore, in order to avoid an adverse effect during firing of the scale, a method of periodically dissolving and removing the scale with an alkali (Japanese Patent Laid-Open No. 6-340411).
Japanese Patent Application No. Hei 9-120750 proposes a method of periodically immersing the inside of a reactor with hot water to modify the scale so that the scale does not become black foreign matter during firing. However, in the method of dissolving and washing with alkali, since several batches are contaminated with a trace amount of alkali after washing, it cannot be used for applications requiring an extremely low level of alkali content such as semiconductor applications,
It is industrially problematic to carry out it too frequently because it causes product loss. On the other hand, as described above, there is no product loss in the scale reforming by washing with hot water inside the reactor, but there is a problem that the reforming effect is extremely small in a portion not in contact with hot water.

Further, when the degree of malignancy of the scale formed in the portion not in contact with the hot water to black foreign matter was examined, a reduced pressure suction pipe for performing evacuation after hydrolysis reaction and gelation, It has been found that the degree of malignancy of the scale generated on the inner surface of the vent line which is open so that the inside of the system is not pressurized is extremely high. The reason for this is
It is considered as follows. That is, since the hydrolysis reaction of alkoxysilane and water is an exothermic reaction, when the reaction is carried out on an industrial scale, the internal liquid temperature is usually from several tens of degrees Celsius,
It rises to the boiling point in the hydrolyzate composition. At this time, the evaporated alkoxysilane and water are condensed in a pipe provided in a gaseous phase portion, and a part thereof is gelled and deposited on the inner surface of the pipe to form a scale. By repeating this phenomenon for each batch, it is considered that the degree of malignancy of the scale against black foreign matter progresses.

Therefore, in the present invention, in the step of hydrolyzing the alkoxysilane, the portion where the evaporated component condenses is cooled to 10 ° C. or less. By doing so, even in a hydrolysis reactor having a permanent pipe in the gas phase such as a reduced pressure suction line and a vent gas line, if the pipe installed in this gas phase is cooled to 10 ° C. or less, the above-mentioned condition is obtained. Can significantly reduce the scale problem of Cooling temperature is 1
A temperature of 0 ° C or lower is sufficient for achieving the effects of the present invention, but is preferably 5 ° C or lower. Although the cooling method is not particularly limited, for example, a method in which a pipe has a double structure (jacket structure) and a refrigerant is caused to flow through an outer jacket portion, or the like can be considered. If the temperature is higher than 10 ° C., the condensate of the evaporating component gels and scale is generated, which is not preferable. Although the mechanism of suppression of scale formation by cooling is not necessarily clear, generally, the gelation time of the alkoxysilane hydrolyzate requires a longer time as the temperature is lower. It is considered that the entire amount of the condensate is refluxed into the reactor without gelling. The cooling portion may be at least a part of a portion where the evaporation component can condense. The location where the evaporated components condense depends on the structure of the reactor, but typically includes the above-described permanent piping in the gas phase. According to the method of the present invention described above, generation of a malignant scale in the condensate adhering portion of the reaction gas phase can be almost completely suppressed.

Next, gelation of the hydrolysis product is carried out under heating or at room temperature. When heating is performed, the rate of gelation can be increased, so that the gelation time can be adjusted by adjusting the degree of heating. In order to obtain silica gel, any of these known methods can be adopted without particular limitation, but generally, the whole is formed into one gel rather than forming silica gel as a precipitate, and this is crushed and crushed. The silica gel powder is preferably subjected to the following steps because the resulting quartz powder has particularly excellent physical properties, does not require extra steps such as separation from supernatant, and has a good yield.

The silica gel thus obtained is subdivided by grinding or the like, if necessary, to obtain a silica gel powder. In general, the silica gel is dried prior to the calcination described below. In this case, the gel may be finely divided and then dried, or dried and finely divided. In any case,
The particle size after drying is 10-1000 μm, preferably 10
Subdivision is performed so as to be 0 to 600 μm, and the average particle size is set to 150 to 300 μm. The drying temperature varies depending on the conditions, but is usually 50 to 200 ° C. The operation can be performed either batchwise or continuously. The degree of drying is usually carried out to a liquid content of about 5 to 30% by weight. The dried silica gel powder thus obtained is fired. At this time, the dried silica gel powder can be directly heated in a temperature range of 1000 to 1300 ° C. to make it nonporous to obtain a synthetic quartz powder.
It is desirable to perform the heat treatment at 1000 to 1300 ° C. after performing the heat treatment at a temperature of 00 to 500 ° C. to sufficiently reduce the carbon concentration. This is for the following reason.

In the dry silica gel powder obtained by the hot water immersion treatment according to the above method, usually, a small amount of unreacted alkoxy groups and by-product alcohol remain. Actually, when the carbon concentration in the silica gel powder is measured, it is from several hundred ppm to 2%, depending on the drying conditions.
This silica gel powder is put in an oxygen-containing gas at 1000 to 13
When heated in a temperature range of 00 ° C. to make the pores non-porous, most of the carbon is burnt and removed in the process of raising the temperature, but part of the carbon may be trapped as unburned carbon in the synthetic quartz powder. It has been clarified by the present inventors that the use of synthetic quartz powder containing fuel carbon causes CO2 or CO gas during melt molding and causes the generation of bubbles (Japanese Unexamined Patent Publication No. Hei 9-1997). No. 118513). Therefore, it is necessary to remove substantially all of the unburned carbon before sealing the silica gel. Here, according to the further study of the present inventors, when the residual carbon in the dried silica gel powder is brought into contact with an oxygen-containing gas in a temperature range of 300 ° C. or more, the residual carbon is reduced while generating CO and CO2, and the treatment temperature is increased. It has been clarified that the rate of decrease increases as the temperature increases, and that the nonporous silica gel rapidly proceeds in a temperature range of 600 ° C. or higher. Therefore, after the carbon concentration is reduced to several hundred ppm or less by heat treatment at a temperature of 300 to 500 ° C., as described below, 1000 to 1300 ° C.
It is desirable to be subjected to a heat treatment.

The heating operation in the temperature range of 1000 to 1300 ° C. is generally performed by a batch method. The container used is made of a material that is heat-resistant and does not cause contamination of synthetic quartz powder with impurities, for example, a crucible made of quartz. In the present invention, the firing step described above is performed while flowing a dry gas. Examples of the drying gas include an inert gas such as nitrogen and argon, oxygen, air, and a mixed gas thereof, but air is preferred from the viewpoint of economy. The gas to be used is sufficiently removed from the water contained by a method such as adsorption and the dew point is kept at -20 ° C or lower, preferably -40 ° C or lower. If the dew point is higher than this range, the residual silanol concentration of the obtained synthetic quartz powder is undesirably high.

It is convenient to distribute the drying gas by circulating it on the surface of the powder layer filled in the container, or by supplying it from an insertion tube inserted into the powder layer. Among these methods, the method of supplying with an insertion tube inserted into the powder layer is advantageous because a synthetic quartz powder having uniform characteristics can be obtained even when firing is performed using a large-capacity container. However, when the gas is supplied from the insertion tube in the powder layer, the gas flow rate is desirably selected from a region where the powder does not exhibit the bubbling phenomenon. When the bubbling phenomenon occurs, although the reason is not well understood, the rate of reduction of silanol is reduced, and further, powder is spilled out of the container. When the bubbling phenomenon starts, the powder near the dry gas inlet starts to dance on the surface of the powder layer, and when the flow rate is further increased, large bubbles burst as if the bubbles in the so-called "blood pond hell" burst. Since it flutters, it can be easily confirmed that the bubbling phenomenon has occurred.

The material of the insertion tube is preferably a material which does not generate impurities such as quartz, as in the case of the container.
Thus, the silanol concentration in the synthetic quartz powder was 100 ppm
Hereinafter, the sintering is continued until it becomes preferably 60 ppm. The synthetic quartz glass powder thus obtained is melt-molded by a known method to obtain a quartz glass molded body. The molding method may be appropriately selected according to the use of the molded article. For example, if the use is a crucible, the arc melt method,
As the jig for C, a method of once forming an ingot by the Bernoulli method using an oxyhydrogen flame, a fusion method of heating and melting under vacuum using a carbon mold, and the like are exemplified.
The synthetic quartz glass powder obtained by the present invention suppresses the generation of black spots in the powder after firing, and further includes a silicon single crystal pulling crucible manufactured using the synthetic quartz glass powder, a tube and a jig of a diffusion furnace, and the like. It can be suitably used as a raw material for obtaining a high quality glass molded product with few bubbles in glass products for optical communication and optical fields such as quartz glass members for semiconductor production and optical fibers.

[0020]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. (Example 1) 30 kg of ultrapure water was charged into a jacketed horizontal cylindrical reactor having an inner volume of 200 liters having a ribbon type stirring blade and schematically shown in FIG. 1, and then stirring was started at 20 rpm. Thereafter, 50 kg of tetramethoxysilane was charged over 3 minutes, and the reaction was started with the vent gas line made of SUS304 in FIG. 1 open. About 50 cm from the end of the vent line on the side of the reactor, a jacket structure was used, and a coolant cooled to 2 ° C. was always passed through the jacket. In FIG. 1, 1 is a jacket portion,
2 is a stirring shaft and 3 is a discharge valve.

The temperature of the hot water flowing through the jacket of the reactor body was 45 ° C. When a uniform sol was obtained, stirring was stopped, and the contents were allowed to stand for 30 minutes. When the change in the contents was observed through a glass window of the reaction vessel, gelation occurred 30 minutes after the completion of the charging. Thereafter, the pressure in the system was reduced by a vacuum pump through the reduced pressure suction line in FIG.
After reaching 04 Pa, the pressure in the system was restored with nitrogen gas. This operation caused cracks in the integral gel block. In addition, a jacket structure was formed at 50 cm from the end of the reduced-pressure suction line on the reactor side, and a 2 ° C. refrigerant was always circulated through the jacket portion. After that, the rotation of the stirring blade was started again to roughly pulverize the gel to a size not larger than the fist size, and then the discharge valve provided at the bottom of the reactor was opened to take out the bulk gel from the reactor.

Next, after closing the discharge valve, the reaction was repeated in the same procedure. In addition, every time 15 batches of the reaction were completed, 170 liters of ultrapure water was charged into the reactor, and while flowing steam through the jacket portion, the reactor was boiled with the vent line open, and held for 5 hours in the boiling state.
Perform hot water immersion treatment of the scale attached inside the reactor,
An operation of extracting hot water from the bottom of the reactor was performed. When the inside of the reactor was observed after the completion of 151 batches, it was observed that a white scale was adhered to the inner wall of the reactor (material: SUS304). There was virtually no scale development on the inner surface. In addition, a small amount of the scale formed on the inner wall of the reactor is collected, dried, and crushed.
Into an electric furnace in the air atmosphere maintained at
After holding for 0 minutes, the degree of discoloration was visually observed, but all the particles were transparent and no blackened portion was observed.

On the other hand, the gel of the 151st batch extracted from the reactor was pulverized by a SUS304 co-mill type pulverizer, and the obtained powdery gel was subjected to a 250-liter conical dryer (rolling drier). , And dried while reducing the pressure by flowing steam through the jacket to obtain about 22 kg of a silica gel powder. The liquid content of a part of the obtained silica gel powder was measured.
It was 11% by weight on a loss on ignition basis. Subsequently, the silica gel powder was classified and adjusted to a particle size of 106 to 500 μm.

15 kg of this classified silica gel product is put in a quartz glass container, and while flowing dry air having a dew point of -45 ° C., is heated to 500 ° C. using a box-type electric furnace at 250 ° C.
/ Hr, heat up at 500 ° C for 5 hours, and furthermore 500 ~ 12
The firing was performed under the condition of raising the temperature to 100 ° C. at 100 ° C./Hr and maintaining the temperature at 1200 ° C. for 40 hours. After cooling, while taking out the fired product, a visual inspection of the amount of black foreign matters was performed, and the occurrence of black spots was zero during the taken-out amount of 14 kg. In addition, the quartz glass powder obtained after the firing was melted by a Bernoulli melting method (oxyhydrogen flame melting method) to produce an ingot of 10 mmφ × 50 mmL and observed, but no bubbles were generated.

(Comparative Example 1) The same operation as in Example 1 was carried out except that the refrigerant was not circulated through the jackets of the vent gas line and the reduced pressure suction line in Example 1 described above, and the reaction of 151 batches was performed. Was done. After that, when the inside of the reactor was observed, it was confirmed that white scale was deposited not only on the inner wall surface of the reactor but also on the inner surfaces of these vent gas lines and vacuum suction line piping. A part of these scales was collected, dried, pulverized, put into an electric furnace in an air atmosphere maintained at 1000 ° C., and maintained for 10 minutes. The degree of discoloration was visually observed. As for the scale of the inner wall portion, no blackened portion was observed at all, but the scale in the vacuum suction line and the vent gas line piping was mostly blackened.

On the other hand, the gel of the 151st batch discharged from the reactor was treated under the same conditions as in Example 1. Then, the amount of black foreign matter generated after the completion of the firing at 1200 ° C. was visually inspected.
Five black spots were confirmed. Further, a part of the calcined quartz glass powder was melted by the Bernoulli method, and 10 mmφ × 5
When a 0 mmL ingot was produced, two bubbles having a size of 20 μmφ or more were confirmed.

[0027]

According to the present invention, it is possible to obtain a synthetic quartz glass powder in which the generation of black foreign matter is suppressed, and it is possible to obtain a quartz glass molded body having few bubbles.

[Brief description of the drawings]

FIG. 1 is a schematic view of a jacketed horizontal cylindrical reactor used in Example 1.

[Explanation of symbols]

 1 Jacket part 2 Stirring shaft 3 Discharge valve

Claims (2)

[Claims] The present invention is characterized in that, when a gel obtained by hydrolyzing an alkoxysilane is calcined to obtain a synthetic quartz glass powder, in a hydrolysis step, a portion where an evaporating component condenses is cooled to 10 ° C. or less. Of producing synthetic quartz glass powder. 2. A method for producing a quartz glass molded body, comprising melt-molding the synthetic quartz glass powder obtained by the method according to claim 1.