Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Patent
1997-11-26
2000-10-10
Dunn, Tom
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
423335, 501 54, 65 172, 65 331, 65440, C10B 3312
Patent
active
061298999
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to efficient processes for producing a synthetic quartz powder and a shaped product of quartz glass.
BACKGROUND ART
In recent years, for glass products to be used in the field of optical communication or in the semiconductor industry, a very strict control has been carried out with respect to minor impurities and fine bubbles in the products. Such high quality glass is produced mainly by e.g. (1) a method for purifying natural quartz, (2) a method wherein a fume formed by decomposition of silicon tetrachloride in an oxyhydrogen flame is deposited and grown on a substrate, or (3) a method wherein a silica gel obtained by hydrolysis and gelation of a silicon alkoxide or the like, is baked to obtain a synthetic quartz powder, and such a quartz powder is fused to obtain a shaped product.
However, the method (1) has a problem that there is a limit in reduction of the content of minor impurities, and the method (2) has a problem that the production cost is extremely high. On the other hand, the method (3) of baking a silica gel does not necessarily satisfy the required level, although as compared with the method (2), it is possible to obtain a synthetic quartz powder having a low content of minor impurities at a low cost. Besides, this method has a problem that fine bubbles may sometimes form in the shaped product as the final product, and such fine bubbles tend to cause various troubles.
The present inventors have conducted extensive studies to find out a solution to the problem in the above method (3) for producing a synthetic quartz powder by baking a silica gel, i.e. to find out a method for producing a synthetic quartz powder whereby formation of fine bubbles in a shaped product obtainable by fusing it is very little, and a method for carrying out such production industrially advantageously, and the following points have been made clear. Namely, baking of a silica gel is carried out by charging a silica gel into a quartz container in order to avoid contamination of impurities from the container and heating it in e.g. an electric furnace. Especially for industrial production, a quartz crucible having a large diameter or the like is used. However, a silica gel has a low bulk density as compared with the quartz powder, and accordingly, the container used for baking can not effectively be utilized, whereby the productivity is poor, and the production cost is high. Accordingly, to improve the productivity, it is important to increase the bulk density of the powder to be charged into the crucible.
On the other hand, in the production of a shaped product employing a quartz powder, formation of fine bubbles during the production of the shaped product, is influenced by the temperature-raising process in the baking step for the production of the quartz powder. In the silica gel powder obtained by hydrolysis of a silicon alkoxide, unreacted alkoxy groups and a part of an alcohol formed by a side reaction will remain even if the alcohol formed by a side reaction has been removed by drying. In fact, when the carbon concentration in the silica gel powder dried, is measured, it is from 1 to 3%, although it varies depending upon the drying condition. If this silica gel powder is baked in an oxygen-containing gas, the majority of carbon can be removed by combustion during the temperature-raising process. However, a part thereof may sometimes be included in the synthetic quartz powder as non-combustion carbon. If this synthetic quartz powder containing such non-combustion carbon, is used, CO or CO.sub.2 gas will be formed during melt-forming, which causes formation of bubbles. Accordingly, it is necessary to remove substantially the entire amount of non-combustion carbon before the pores of the silica gel be closed, and the temperature-raising speed in the temperature-raising process becomes important.
However, as mentioned above, when industrial production of a synthetic quartz powder is intended, a quartz crucible having a large diameter is required, and the temperature in
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Katsuro Yoshio
Maeda Hiroshi
Oishi Shoji
Shimoyama Masaru
Dunn Tom
Mitsubishi Chemical Corporation
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