Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Patent
1997-11-04
2000-08-29
Group, Karl
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
501 12, 423325, 423338, C03C 306
Patent
active
061108529
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a production of a synthetic quartz glass powder of high purity and low silanol content.
BACKGROUND ART
In recent years, glass products used in the industry of optical communication, semiconductors, etc. are under very severe controls with respect to the purity of the constituting glass materials.
To produce such a highly pure glass product, there are mainly the following methods: (1) a method using sand-like natural quartz powder (what is called "sand") obtained by pulverizing natural quartz; to obtain more purified ones, (2) oxygen-hydrogen flame method, a method which comprises using a lump of fume, obtained by adhesion and growth on the substrate, of the fume generated by the hydrolysis of silicon tetrachloride in the oxygen-hydrogen flame; (3) a method whereby an alkoxysilane is hydrolyzed and gelled to obtain a silica gel powder, and then the silica gel powder is calcined to obtain a glass powder, which is then fused and shaped. However, among these methods, method (1) has limit in lowering extremely small amount of impurities content, method (2) has problem of expensive production cost.
Method (3), using silica gel, especially using silicon alkoxide as the starting material, quartz glass with low impurity content can be economically obtained compared to the method (2). However, by this method, silanol groups which the starting material has during the hydrolysis and gelation, will remain as residual silanol groups in the particles after the calcination, whereby, as compared with natural quartz powder, the product has a higher silanol content. If the silanol content is high, the viscosity at a high temperature tends to be low, and such a product is not useful for e.g. a crucible for drawing a silicon single crystal or for a diffusion furnace semiconductors (Japanese Unexamined Patent Publication No. 320232/1989). For such applications, it is said that the silanol content is preferably 100 to 50 ppm.
However, there is also such a problem that lowering residual silanol content to this level is technically difficult in industrial scale, and even when attained, long time calcination is necessary. If lowering of silanol content is not enough, there is also such a problem that the obtained shaped quartz glass product has small bubbles.
To lower the silanol concentration efficiently, there are also the following methods. One comprises calcining silica gel in a gas containing halogen dehydroxy agent such as chlorine(Japanese Unexamined Patent Publication No. 186232/1986). However, by this method, although silanol is efficiently reduced, the halogen atom remains, so the quality of the obtained shaped product cannot be satisfactory.
To reduce silanol content, there are also a method which comprises selecting a low water vapor partial pressure atmosphere for calcination (Japanese Unexamined Patent Publication No. 289416/1990), and a method in which calcination is carried out by introducing dry gas in the silica gel (Japanese Unexamined Patent Publication No. 83711/1992). By the former method, silanol is reduced by a long time calcination, but it can not necessarily shorten the calcination time. Namely, long time calcination is necessary. It is described that the latter method improves the silanol reducing rate, and it is explained that a precipitated silica gel having an average particle size of 120 .mu.m is used as a starting material, and a gas with low water vapor partial pressure is used, then the silanol content after the calcination can be reduced to no more than 60 ppm. By the former method, for example under high temperature of 1100 to 1250.degree. C., calcination is carried out by introducing a dry gas, and a synthetic quartz glass powder is obtained, then the obtained synthetic quartz glass powder is took out of the furnace and cooled. By such methods, the silanol content in he obtained product is from 60 to 100 ppm.
As described above, in the conventional methods, the attention was paid to the atmospheric moisture at high temperature, especially not
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Endo Hozumi
Katsuro Yoshio
Nagai Hiroaki
Oishi Shoji
Utsunomiya Akira
Group Karl
Mitsubishi Chemical Corporation
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