Silica glass has been recognized to be very advantageous to many uses, such as crucibles, boards and quartz tubes of furnaces in the manufacturing process of semiconductor, since it has been possible to prepare a silica glass of high purity. Besides, silica glass is used for a glass equipment such as a beaker for chemistry and a cell for optical measurement. Further, the development of the silica glass having less hydroxyl group or having good optical uniformity permits the various optical uses of silica glass. Especially, attention is given to silica glass fiber for optical communication. Furthermore, silica glass is used for a substrate of a Thin Film Transistor. In this way, silica glass is widely applied in various fields and the demand thereof is expected to increase still more in future.
At present, silica glass is mainly obtained by the following three approaches.
Method (1) wherein;
Cleanly refined pure quartz is washed and melted.
Method (2) wherein;
SiO.sub.2 is formed from raw material SiCl.sub.4 or SiH.sub.4 of high purity.
Method (3) wherein;
a pure silica sand is melted.
Whichever method is used, however, prepared silica glass amounts to expensive due to the high temperature treatment which is inevitably included in the manufacturing process and the difficulty in manufacturing glasses its own.
Accordingly, more economical process is eagerly desired for preparing silica glass. In order to prepare silica glass cheaply, two approaches are practiced; one is a sol-gel technology wherein a metal alkoxide is used as raw material, the other is a sol-gel technology wherein fumed silica is used as raw material. Each approach is described hereinafter.
The sol-gel process for economically preparing silica glass from metal alkoxides is studied by Nogami and Moriya, in Journal of Non-Crystalline Solids, 37, (1980) pp. 191-201.
The steps of this process are as follows; Silicon alkoxide, water, alcohol and proper catalyst such as hydrochloric acid or ammonia are mixed together and hydrolyzed. After gelling the hydrolyzed solution, the gel is dried to form dry gel. Dry gel is sintered by increasing temperature and collapsed completely to form silica glass. According to this sol-gel process, silica glass allowed to be highly pure for an alkoxide used as raw material is easily purified. In addition, manufacturing cost is less expensive than the conventional process for heat treatment is carried out at low temperature.
On the contrary, this process suffers the deficiency that large articles of dry gel tend to crack during drying and similarly during heating in a process of conversion from dry gel to silica glass. Under the condition, the obtained silica glass is not sufficiently large to put into practical use. So far as known from references, a disc type silica glass of 28 mm.phi. obtained by the study of Nogami and Moriya seems to be the largest at present.
On the other hand, the sol-gel process in which fumed silica is used as raw material is disclosed in Journal of Non-crystalline solids, 47, (1982) pp. 435-439 by E. M. Ravinovich et al. Fumed silica (trademark: Cab-o-sil supplied by Cabot Corp.) is mixed with water to be a hydrosol. After the hydrosol is gelled, the gel is dried to form dry gel, which is converted to silica glass by sintering. Being different from said metal alkoxide process, this sol-gel process using fumed silica has an advantage that the dry gel is hard to be broken or cracked during drying and sintering, which permits the formation of fairly large silica glass. Nevertheless, a realized silica glass in the aforesaid reference is at most as large as 95.times.15.times.5 mm (including a 4% B.sub.2 O.sub.3), which appears to be maximum in size. Moreover, this sol-gel method is disadvantageous in that the bubbles are so easily generated that there is a lot of cells in a realized silica glass. Therefore, this sol-gel process is unsuitable for an application requiring an optical uniformity.
As mentioned above, the conventional sol-gel process has not been enough sufficient to be applied in various field, since prepared silica glass thereby is not so large nor so good in quality.