Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1994-12-30
2001-10-02
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C065S021100, C065S033100
Reexamination Certificate
active
06296826
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for the preparation of vitrified silica particles. More particularly, the invention relates to a method for the preparation of vitrified silica particles suitable as a base material of transparent fused silica glass bodies from extremely fine silica particles having no usefulness as such due to fluffiness to cause difficulties in handling.
As is known, synthetic fused silica glass is prepared by several different methods including the so-called direct method, soot method and sol-gel method in a gross classification. In the so-called direct method, a vaporizable silicon compound is introduced into an oxyhydrogen flame in which the silicon compound such as silicon tetrachloride is flame-hydrolyzed and converted into fine silica particles to be deposited on a rotating substrate body to form a porous silica body which is vitrified in situ as it grows by heating with the oxyhydrogen flame. This method is widely practiced in the preparation of fused silica glass bodies useful in the manufacture of various kinds of optical parts because the fused silica glass body prepared by this method has a high purity and is free from occlusion of bubbles.
In the soot method, on the other hand, the porous silica body formed by the deposition of the silica particles, i.e. silica soot, produced from a silicon compound in an oxyhydrogen flame on the rotating substrate is, without being vitrified in situ as it grows, vitrified in a separate electric furnace into a transparent silica glass body. Because the conditions for the flame hydrolysis of a silicon compound can be moderate as compared with the direct method, the soot method is practiced in the preparation of a preform as a precursor of optical fibers which must have a modified refractive index by doping with a refractive index-modifying dopant such as germanium and the like. Further, the porous silica glass body before sintering and vitrification can be subjected to a dehydroxylation treatment or doping with fluorine and other vaporizable dopants by utilizing the porosity thereof.
The sol-gel method is a wet process in which a silicate ester such as tetraethoxy silane is subjected to a hydrolysis-condensation reaction to form a block of semi-solid sol which is then dried and sintered at a high temperature into a vitrified transparent silica glass body. A problem in this method is that formation of cracks is sometimes unavoidable in the block of the semi-solid gel so that this method is hardly applicable to the preparation of a large silica glass body. Accordingly, this method is rarely utilized in the preparation of fused silica glass articles and the only way to industrially practice this method is for the preparation of a silica filler as a compounding material of molding resins and the like by pulverizing the sintered body.
A problem common in the direct and soot methods is the low yield in the deposition of the silica particles relative to the amount of the consumed vaporizable silicon compound. Namely, the yield in the direct method is usually about 30% and the yield in the soot method also can rarely exceed 50% with a great loss of the silicon compound due to the loss of the silica particles formed therefrom but dissipated or discharged out of the oxyhydrogen flame furnace without being deposited to form the porous silica body. The silica particles recovered in a filter bag and the like as a flue dust have a very small particle diameter of 0.2 &mgr;m to several micrometers in the form of a very fluffy powder difficult to handle. The thus recovered silica powder as such has almost no industrial usefulness due to the uncontrolled particle size distribution and possible contamination and must be discarded or disposed as an industrial waste material sometimes to cause serious environmental pollution.
As a consequence of the above mentioned problem in the prior art methods for the preparation of fused silica glass in which a half amount or even larger of the starting silicon compound is consumed futilely, the cost for the preparation of synthetic fused silica glass is increased so much even if not to mention the problem in respect of the environmental pollution. Thus, it is an important problem in the technology for the preparation of synthetic fused silica glass to establish an efficient way for the effective utilization of the extremely fine fluffy silica particles produced in the process of flame hydrolysis of a vaporizable silicon compound and heretofore discarded as an only way for the disposal thereof.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide an efficient way for the effective utilization of extremely fine silica particles produced in the manufacturing process of synthetic fused silica glass or in the handling thereof and utilized or disposed with great difficulties due to the fluffiness and impurities contained therein. Another object of the present invention is to provide a method for the preparation of poreless vitrified silica glass particles suitable for use in the preparation of a transparent fused silica glass body starting from extremely fine silica particles produced in the manufacturing process of synthetic fused silica glass or in the handling thereof and discarded heretofore as an only way for the disposal thereof.
Thus, the method of the present invention for the preparation of poreless vitrified silica glass particles comprises the successive steps of:
(a) admixing finely divided silica particles having a specific surface area in the range from 1 m
2
/g to 400 m
2
/g or, preferably, from 10 m
2
/g to 400 m
2
/g with water to form a uniform mixture of the silica particles and water;
(b) dehydrating and drying the mixture of the silica particles and water to give a porous caked mass of the silica particles;
(c) disintegrating the porous caked mass of the silica particles to give porous silica particles;
(d) heating the porous silica particles at a temperature in the range from 800° C. to 1300° C. for a length of time in the range from 5 hours to 100 hours to give semi-sintered silica particles; and
(e) heating the semi-sintered silica particles at a temperature in the range from 1350° C. to 1550° C. for a length of time of at least 10 minutes.
When purification of the product silica glass relative to the content of metallic impurities is desired, the porous silica particles obtained in the step (c) is subjected to washing or leaching with an aqueous acid solution so as to dissolve way any metallic impurities contained in the particles followed by thorough rinse with water.
When doping of the product silica glass with a dopant is desired, it is preferable to soak the porous silica particles obtained in the step (c) with a solution containing the dopant compound followed by drying before the step (d).
Further, when it is desired to decrease the content of hydroxyl groups in the product silica glass, it is preferable to conduct the heating treatment in the step (d) in an atmosphere containing chlorine gas, thionyl chloride gas or a combination thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the inventive method basically comprises the essential steps (a) to (e) and, when these steps are undertaken adequately, the fluffy silica powder consisting of extremely finely divided silica particles recovered as a flue dust can be converted into poreless vitrified silica particles of a relatively large particle size easy to handle and suitable as a base material for the preparation of transparent fused silica glass articles.
The starting material used in step (a) of the inventive method is a fine silica powder obtained by collecting the flue dust in the manufacturing process of porous silica glass body by the direct method or soot method although silica powders from any other sources can be used provided that they have equivalent physical properties. Such a finely divided silica powder usually has a particle diameter in the range, for example, from 0.2 to 2 &mgr;m and the bulk density of the po
Fujinoki Akira
Sugama Akihiko
Yokota Tohru
Nguyen Ngoc-Yen
Shin-Etsu Quartz Products Co. Ltd.
Wenderoth , Lind & Ponack, L.L.P.
LandOfFree
Method for the preparation of vitrified silica particles does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for the preparation of vitrified silica particles, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for the preparation of vitrified silica particles will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2588726