Coating processes – Particles – flakes – or granules coated or encapsulated – Inorganic base
Reexamination Certificate
1999-07-22
2001-11-20
Chen, Bret (Department: 1762)
Coating processes
Particles, flakes, or granules coated or encapsulated
Inorganic base
C427S255390
Reexamination Certificate
active
06319550
ABSTRACT:
The invention relates to a method of treating granules of silica made up of mutually agglomerated silica soot particles, in which the granules are placed in a crucible within a furnace which contains an inert gas mixed with a chlorine-containing compound, and in which the temperature lies in the range 1000° C. to 1500° C.
BACKGROUND OF THE INVENTION
Silica granules are used in conventional manner to build up an optical fiber preform, in particular when it is desired to manufacture a monomode optical fiber. The build-up operation is performed, for example, on a bench that is fitted with a plasma torch which is fed with silica granules which are projected onto the preform until a desired diameter has been built up.
Granules obtained by agglomerating silica soot particles are of relatively low density and can contain impurities, coming from the soot particles themselves or from the reactor in which agglomeration took place.
It is known to purify and densify granules prior to using them in a bench for building up a preform. At present, the granules are placed in a dense crucible of opaque silica of very high purity so as to avoid any risk of the granules being contaminated by the crucible. Thereafter, the crucible is placed in a furnace containing an atmosphere of inert gas mixed with a chlorine-containing compound and having a temperature lying in the range 1000 degrees Celsius (° C.) and 1500° C. The chlorine-containing compound diffuses into the granules where it reacts with alkaline or metallic impurities, e.g. by forming gaseous chlorides which are then evacuated from the furnace. Under the action of temperature, the granules densify individually. The crucible is removed from the furnace after a length of treatment time that corresponds to the grains beginning to densify between one another, which densification should be minimized.
The known method of purifying and densifying silica granules is not without its drawbacks.
Firstly, the chlorine-containing compound comes into contact with the granules only via the open portion of the crucible, since the dense walls thereof are gasproof. As a general rule, the crucible is cylindrical in shape and has an open top end which forms a surface via which the chlorine-containing compound diffuses amongst the granules. At given partial pressure for the chlorine-containing compound in the gaseous atmosphere of the furnace, the time required for the chlorine-containing compound to diffuse amongst the granules over the full height of the crucible turns out to be quite long.
Also, although the pure silica crucible does not present any risk of contaminating the granules, it does have chemical affinity for granules made up of silica soot particles, and under the temperature conditions of the oven, that leads to the granules adhering to the walls of the crucible. In practical terms, adhesion makes it difficult to recover the granules in simple manner, and any mechanical action runs the risk of adding contamination.
OBJECT AND SUMMARY OF THE INVENTION
The object of the invention is to treat granules made up of mutually agglomerated silica soot particles by means of a high temperature heat treatment method under a chlorine-containing atmosphere, in which the crucible used does not suffer from the drawbacks due to limited diffusion of the chlorine-containing compound or to adhesion of the granules to its walls.
To this end, the invention provides a method of treating silica granules made up of mutually agglomerated silica soot particles, in which method the granules are placed in a crucible inside a furnace containing an inert gas mixed with a chlorine-containing compound and in which the temperature lies in the range 1000° C. to 1500° C., wherein a crucible is used that is made of porous graphite.
The use of a crucible made of porous graphite serves firstly to increase the surface area of the crucible through which the chlorine-containing compound diffuses. Exchange takes place more quickly between the granules and the gas for given partial pressure of the chlorine-containing compound, and as a result purification is improved.
In addition, the difference in chemical nature between the granules and the crucible makes the two materials inert relative to each other so that the granules do not adhere to the walls of the crucible.
It will be observed that the graphite crucible presents good resistance to abrasion so there is no risk of the granules becoming contaminated during operations of filling and emptying the crucible. Nevertheless, if necessary, it is possible to use a graphite-graphite composite crucible which presents higher resistance to abrasion than does a crucible made of ordinary graphite.
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Campion Jean-Florent
Francois Jean-Philippe
Lavallade Christelle
Oskeritsian Philippe
Saugrain Jean-Maxime
Alcatel
Chen Bret
Sughrue Mion Zinn Macpeak & Seas, PLLC
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