Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-01-19
2001-11-13
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S017300, C065S017400, C065S144000, C065S302000, C065S378000, C065S414000, C065S416000, C065SDIG008, C501S900000
Reexamination Certificate
active
06314766
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of fused silica glass, and, in particular, to methods and apparatus for improving the homogeneity of such glass, i.e., for reducing variations in the index of refraction of the glass.
DESCRIPTION OF THE TECHNOLOGY
FIG. 1
shows a prior art furnace
100
for producing fused silica glass. In overview, high purity fused silica glass is made by depositing fine particles of silica in a refactory furnace at temperature exceeding 1650° C. The silica particles are generated in a flame when a silicon containing raw material along with natural gas is passed through a burner into the furnace chamber. These particles are deposited on the hot surface of a rotating body where they consolidate into a very viscous fluid which is later cooled to the glassy (solid) state. The rotating body is in the form of a refractory cup or containment vessel which is used to provide insulation to the glass as it builds up, so that the furnace cavity formed by the cup interior and the crown of the furnace is kept at high temperatures. In the art, glass making procedures of this type are known as vapor phase hydrolysis-oxidation processes or simply as flame hydrolysis processes. The body formed by the deposited particles is often referred to as a “boule” and this terminology is used herein, it being understood that the term includes any silica containing body formed by a flame hydrolysis process.
The furnace
100
includes a crown
12
having multiple deposition burners
14
, a ring wall
160
which supports the crown, and a rotatable base
18
mounted on an oscillation table
20
. The base
18
is rotatable about an axis
3
, and the table
20
oscillates in a x-y direction in a plane perpendicular to the axis
3
. The crown, ring wall, and base are each made of suitable refractory materials. Preferred patterns for the motion of the x-y oscillation table
20
, which can be used in the practice of the present invention, are described in commonly assigned U.S. Pat. No. 5,696,038, entitled “BOULE OSCILLATION PATTERNS OF PRODUCING FUSED SILICA GLASS”.
A cup or containment vessel
13
is formed on the base
18
by means of a cup wall or containment wall
22
mounted on the base
18
, which forms the cup or containment vessel
13
. The cup or containment wall
22
and the portion of the base
18
surrounded by the wall (the bottom of the vessel) is covered with high purity bait sand
24
which collects the initial silica particles. The wall
22
can be composed of refractory blocks such as outer alumina base block
22
a
and an inner liner
22
b
made of, for example, zironcia or zircon. Other refactory materials and constructions can, of course, be used if desired.
Surrounding the cup wall
22
of the cup or containment vessel
13
is a shadow wall or air flow wall
130
. The shadow wall
130
is mounted on x-y oscillation table
20
by means of feet
140
, e.g., by four feet equally spaced around the circumference of the shadow or air flow wall. Other means of mounting the air flow wall to the oscillation table can be used if desired. In general, the mounting means should include spaces for the ingress of air to the space
175
between the cup or containment wall
22
and the shadow or air flow wall
130
.
Surrounding the shadow wall
130
is a stationary ring wall
160
which supports the crown
12
. A seal
155
is provided between the stationary ring wall
160
and the rotatable and oscillatable shadow or air flow wall
130
. The seal
155
comprises an annular plate
150
which rides in or slides in an annular channel
170
formed within the stationary ring wall
160
. The annular channel
170
can comprise a C-shaped annular metal plate which forms the bottom of the stationary wall, or other forms of motion accommodating seals can be used if desired, including flexible seals composed of flexible metal or refractory cloth which, for example, can be in the form of a bellows.
The furnace of
FIG. 1
employs two gaps around the cup-like containment vessel
13
, including a circumferential gap or passage
175
between the containment wall
22
and the shadow or air flow wall
130
, which gap permits the flow of cooling air along arrows a into the plenum
26
formed between the crown
12
and the vessel
13
. The other gap
165
is formed between the air flow wall
130
and the stationary ring wall
160
, and has a variable dimension resulting from the oscillation of the table
20
, but does not permit the flow of air as a result of the motion accommodating seal
155
. Thus, the air flow around the boule
19
is influenced by the infiltrated air through passage
175
and the inflow from burners
14
.
The products of combustion from burners
14
are exhausted through six ports such as
280
, that are built around the furnace. As noted in
FIG. 1
, the furnace is built such that there are three layers of refractory wall between the glass boule
19
and the ambient air. The innermost wall
22
, which is part of the cup-like vessel
13
, is isolated from the boule by liners
22
b
such as zircon. The second layer of wall, termed the shadow or air flow wall
130
, is separated from the cup-like vessel
13
by a gap of roughly three inches. The outermost layer of wall, termed the stationary ring wall
160
, is further separated from the shadow wall
130
by an air gap
165
that roughly measures four inches. The walls are built to provide a furnace cavity where the temperatures as well as the furnace atmosphere can be maintained. Radial and circumferential uniformity of both furnace atmosphere and the temperature is important because it directly effects the quality of glass. Temperature uniformity is important for providing consistency in glass density and refractive index. Compositional uniformity is important in providing the consistency in glass density and the hydrogen dissolved in the boule
19
.
Although the horizontal steel plate
150
effectively blocks the gap
165
between the ring wall
160
and the shadow wall
130
, the gap
175
between the shadow or air flow wall
130
and the cup containment wall
22
is open so that the ambient air is free to flow from close to the furnace base from the furnace room to the exhaust ports
280
. This air flow through passage
175
and into the chamber or plenum
26
of the furnace is necessary to cool down the steel bands which hold the refractory blocks together forming the cup like vessel
13
. Without the benefit of such cooling air, the steel bands would expand and slip out of retaining grooves resulting in the vessel
13
falling into pieces.
In order to provide effective removal of the products of combustion from the furnace cavity or plenum
26
, the six port boxes
280
are maintained at a negative pressure. Because air is free to move through the circumferential passage
175
between the cup or containment wall
22
and the shadow or air flow wall
130
, the furnace exhaust consists of gases from burners
14
pulled out of the furnace cavity
26
and the air pulled through the gap or passageway
175
as indicated by arrows a. Because of the flow patterns created near the rim of the cup vessel
13
it became clear that a portion of the air that was being pulled up through the passage
175
was in fact entering the furnace cavity
26
. Gas composition measurements carried out at various radial depths from the inner surface of wall
22
, indicated that the products of combustion were being diluted by the infiltrated air, but that the concentration of CO
2
increased as the sampling probe was inserted radially deeper into the furnace cavity. Thus, it became apparent that the effect of the infiltrated air flow through passage
175
on the homogeneity of the boule
19
was that rim portions were influenced to a greater degree than central portions. The adverse effect of the entrained air through passage
175
is twofold. Firstly, it cools the products of combustion near the rim of the vessel
13
and reduces the gas temperatures in such region. Secondly, the effect is the dilution of the furnace atmosphe
Marley Floyd E.
Misra Mahendra K.
Sproul Merrill F.
Colaianni Michael
Corning Incorporated
Nwaneri Angela N.
Turner Burt R.
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