Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Patent
1997-11-04
2000-06-06
Marcheschi, Michael
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
501 54, 65 172, 65440, C03B 1906, C03B 2000
Patent
active
060718382
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a silica gel useful as a starting material for synthetic quartz of high purity and high quality and the production of the same, and a synthetic quartz glass powder, a quartz glass shaped product and processes for producing these.
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 using a gel obtained from an organic metal compound such as alkoxysilane as a starting material, what is called solgel method.
However, these methods all have their merits and demerits. (1) When natural quartz is used as starting material, metal elements e.g. aluminum, iron, are included essentially inside quartz particle, and it is difficult to purify to the level of not more than 10 ppb even by repeating purification such as acid washing. (2) By hydrogen oxygen flame method, using tetrachlorosilane, high purification can be sought. However, as cost has difficulty to compensate industrially, mass production is not achieved.
On the other hand, (3) sol-gel method needs low cost hence mass production can be sought, but quartz glass obtained by sol-gel method generally contains fine bubbles said to derive from silanol, causing problems especially in semiconductor industries e.g. photomask, crucible for single silicon crystal drawing, and also in optical communication fields such as optical fiber causing problems such as optical loss during fiber elongation and during use and bad effects due to gas in the bubbles.
Also, in sol gel method, synthetic quartz glass powder obtained from a gel treated by steam in advance before drying, or a gel washed with water before calcination, can reduce in some degree the bubbling in the shaped glass product after fusing. However, such steam treatment needs a long time, and also there is a problem that, because the gel contains large amount of water, mechanical strength of the gel is weak, and tends to collapse and pulverize during the steam treatment. Also, relating to the water treatment before calcination, repetition of process such as the dried gel is again washed with water and dried, causes waste of energy and problem of severe cost up. As described heretofore, by the conventional methods, there are problems both technically and in view of cost, and in addition to that, the degree of preventing bubbling is not enough, so also from this point of view, these methods couldn't be used in industrial operation.
DISCLOSURE OF THE INVENTION
The present inventors have conducted extensive studies, according to the above mentioned problems. And they have found out that, inexpectedly, bubbles in the obtained glass product derive not only from silanol in the gel, but they also derive from abnormal particles included in the gel. According to the studies of the present inventors, gel obtained by sol-gel method generally has high content of carbon such as from 0.5 to 2 wt %, therefore by directly calcining a gel with such a high carbon content, black spots can be found in the obtained synthetic quartz glass powder, and furthermore bubbles appear when used at a high temperature. The present inventors have also found out that the main source of the black spot here is the scaling material, that is the silica gel formed by sol-gel reaction adhered to the inner wall of the apparatus, which exfoliates, slips out and is included in the gel product. Namely, at the inner wall of the reaction vessel, dryer, piping etc. which are employed in the sol-gel meth
REFERENCES:
patent: 5145510 (1992-09-01), Saito et al.
patent: 5516350 (1996-05-01), Onoda et al.
patent: 5604163 (1997-02-01), Endo et al.
Endo Hozumi
Katsuro Yoshio
Shimoyama Masaru
Utsunomiya Akira
Marcheschi Michael
Mitsubishi Chemical Corporation
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