U.S. patent application number 10/762325 was filed with the patent office on 2005-04-07 for quartz fusion furnace and method for forming quartz articles.
Invention is credited to Ahlgren, Frederic Francis, Giddings, Robert Arthur.
Application Number | 20050072191 10/762325 |
Document ID | / |
Family ID | 32313285 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050072191 |
Kind Code |
A1 |
Giddings, Robert Arthur ; et
al. |
April 7, 2005 |
Quartz fusion furnace and method for forming quartz articles
Abstract
A crucible for melting a silica for fusion of said silica into a
desired shape. The crucible having a main body with inner and outer
surfaces comprised of a refractory material. In addition, at least
a portion of the inner surface includes a barrier layer comprised
of a material selected from rhenium, osmium, iridium, and mixtures
thereof. An inlet tube to the crucible being provided to supply an
oxidizing gas to a melt zone.
Inventors: |
Giddings, Robert Arthur;
(Slingerlands, NY) ; Ahlgren, Frederic Francis;
(Highland Heights, OH) |
Correspondence
Address: |
PHILIP D FREEDMAN PC
P. O. BOX 19076
ALEXANDRIA
VA
22320
US
|
Family ID: |
32313285 |
Appl. No.: |
10/762325 |
Filed: |
August 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10762325 |
Aug 30, 2004 |
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09636286 |
Aug 10, 2000 |
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6739155 |
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Current U.S.
Class: |
65/374.12 |
Current CPC
Class: |
C03B 3/00 20130101; C03B
17/04 20130101; C03B 5/1675 20130101; C03B 5/425 20130101; Y02P
40/57 20151101; C03B 5/12 20130101; C03B 5/021 20130101 |
Class at
Publication: |
065/374.12 |
International
Class: |
C03B 001/00 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. A furnace for melting silica for fusion into a desired shape,
said furnace comprising a body having a melting zone comprising a
refractory material wall with a protective lining selected from the
group consisting of rhenium, osmium, iridium and mixtures thereof
and a drawing zone, said melting zone including a gas feed inlet
for introducing an oxidizing gas.
11. The furnace of claim 10 wherein said melting zone comprises
walls of the refractory material and an inner barrier layer.
12. The furnace of claim 11 wherein said barrier layer comprises a
material selected from the group consisting of rhenium, osmium,
iridium and mixtures thereof.
13. The furnace of claim 10 wherein said refractory material wall
comprises a refractory material selected from the group consisting
of tungsten, molybdenum and mixtures thereof.
14. The furnace of claim 11 wherein said barrier layer provides a
sealed chamber within said refractory material walls, said gas feed
inlet opening into said sealed chamber.
15. The furnace of claim 11 wherein said barrier layer is
physically separated in at least some areas from said refractory
material walls.
16. The furnace of claim 15 including a gas feed inlet for
introducing between the barrier layer and the refractory material
walls.
17. A fused quartz article produced by a method comprising: feeding
a SiO.sub.2 material into a furnace melting zone comprising a
refractory material wall comprising tungsten molybdenum or mixtures
thereof with a protective lining selected from the group consisting
of rhenium, osmium, iridium and mixtures thereof; feeding a gas
mixture comprising at least (1) one inert carrier gas comprising a
member selected from the group consisting of a hydrogen carrier gas
and a noble carrier gas and (2) an oxidizing gas into the
protectively lined furnace melting zone; fusing the
SiO.sub.2material in the protectively lined melting zone of the
furnace in the presence of the gas mixture; and drawing the fused
SiO.sub.2 material from the furnace to form the fused quartz
article. according to the method of claim 1.
18. An optical fiber including a sheath comprising the fused quartz
article of claim 17.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
the production of tubing, rods and the like from crystalline quartz
or other glass like materials. Particularly, this invention relates
to a method and apparatus for use in the production of elongated
quartz members from a silica melt. The present invention is
particularly directed to the manufacture of fused silica tubes for
use in the manufacture of optical fibers.
[0002] Various types of elongated members have been formed
continuously by melting of quartz crystal or sand in an
electrically heated furnace whereby the desired shape is drawn from
the furnace through a suitable orifice or die in the bottom of the
furnace as the raw material is melted. One apparatus for continuous
production of fused quartz tubing, for example, is a tungsten-lined
molybdenum crucible supported vertically and having a suitable
orifice or die in the bottom to draw cane, rods, or tubing. The
crucible is surrounded by an arrangement of tungsten heating
elements or rods which heat the crucible. The crucible, together
with its heating unit, is encased in a refractory chamber supported
by a water-cooled metal jacket. The crucible is heated in a
reducing atmosphere of nitrogen and hydrogen.
[0003] An alternative apparatus provides fused quartz tubing by
feeding natural quartz crystal into a refractory metal crucible
heated by electrical resistance under a particular gas atmosphere
to reduce the bubble content. The bubbles formed by gas entrapment
between crystals and the molten viscous mass of fused quartz do not
readily escape from the molten glass and, hence, remain as bubbles
or lines in the product drawn from the fused quartz melt. By
substituting a melting atmosphere gas which readily diffuses
through the molten material (such as pure helium, pure hydrogen or
mixtures of these gases) the gas pressure in the bubbles was
reduced and thereby the bubble size was reduced. This process uses
a mixture of 80% helium and 20% hydrogen by volume.
[0004] In a further alternative method, a product is obtained by
continuously feeding a raw material of essentially pure silicon
dioxide in particulate form into the top section of an
induction-heated crucible, fusing the raw material continuously in
an upper-induction heat zone of the crucible in an atmosphere of
hydrogen and helium while maintaining a fusion temperature not
below approximately 2050.degree. C. The fused material in the lower
zone of the crucible is heated by separate induction heating means
to produce independent regulation of the temperature in the fused
material. The fused material is continuously drawn from the lower
zone of the crucible through forming means in the presence of an
atmosphere of hydrogen containing a non-oxidizing carrier gas.
[0005] Unfortunately, most of the refractory metal and non-metal
materials used in the crucibles of the above-described apparatus
are undesirable impurities if present in the drawn silica article.
Such refractory material contamination causes discoloration and
occlusions in the silica glass. Furthermore, the presence of
refractory material particles (e.g. 1-10 .mu.m) can degrade the
strength of the resultant silica article. Moreover, the particles
become a flaw in the drawn article that can cause the strand to
break.
[0006] Accordingly, there is a need in the art to reduce
contamination of fused glass occurring from the refractory
materials used in constructing the furnace. This need has increased
recently as semiconductor and fiber optics manufacturing processes,
a primary use for the glass products obtained from the subject
process, have required higher levels of purity and performance.
[0007] Unfortunately, because the furnace is typically constructed
of the refractory materials, the manufacturing plant is usually
contaminated therewith. Accordingly, even a furnace having melting
and drawing zones insulated from refractory materials cannot fully
prevent contamination. It would therefore be desirable to have
available a method for removing and/or reducing the effect of
refractory materials contamination on the strength of the resultant
silica article.
BRIEF SUMMARY OF THE INVENTION
[0008] In an exemplary embodiment of the invention, a method for
forming an elongated fused quartz article is provided. The method
generally comprises feeding a silica or quartz (SiO.sub.2) material
into a furnace. The SiO.sub.2 material is fused in a melting zone
of the furnace under a gas atmosphere including a carrier gas and
at least one oxidizing gas. The article is then drawn from the
furnace.
[0009] In an exemplary embodiment of the invention, a furnace for
melting of the silica and subsequent drawing into a desired shape
is comprised of a body having an outer surface constructed of a
refractory metal and including a inner lining in at least the melt
zone of the furnace of a non-reactive barrier material. The inner
lining is preferably formed of rhenium, osmium, iridium, platinum
or mixtures thereof. Preferably, the furnace will include an inlet
tube for introduction of a carrier gas and an oxidizing gas to the
melt zone.
[0010] The present crucible construction provides a number of
advantages over the prior art. Particularly, furnaces constructed
with rhenium, iridium, platinum and/or osmium lined crucibles
produce products with much lower levels of refractory metal in the
solution. For example, the metal dissolved in the silica can be
reduced to below 10 ppb, preferably below 1 ppb, and preferably
below the current level of detection via NAA. This reduced amount
of refractory metal contamination in the silica melt improves the
chemical composition of the silica glass allowing for a decrease in
discoloration and surface haze. Furthermore, utilization of a
furnace equipped with a crucible including the non-reactive lining
allows operation at optimum temperature ranges. Operation at these
optimum temperatures may achieve better fining. Moreover, operation
at optimum fusion temperatures will increase solubility of gaseous
species in the raw material, thus reducing airline defects in the
drawn products.
[0011] Similarly, the present inventive crucible will also help to
further reduce the presence of haze and discoloration in the
resultant glass products. In addition, the present inventive
furnace allows for the use of an oxidizing atmosphere in the melt
zone. Previously, oxidizing agents in the melt zone were avoided
because of their negative impact on the refractory walls of the
crucible, particularly on tungsten and molybdenum.
[0012] It should be noted that the terms "quartz" and "silica" are
used interchangeably throughout this application, both being
directed generally to the compound SiO.sub.2. Nonetheless, the
present invention encompasses the use of any raw material
introduced to the melting furnace, including but not limited to
natural silica/quartz and synthetic silica.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The structure, operation and advantages of the present
preferred embodiment of this invention will become further apparent
upon consideration of the following description, taken in
conjunction with the accompanying drawings, wherein:
[0014] FIG. 1 is a longitudinal sectional view of a furnace of the
present invention;
[0015] FIG. 2 is a schematic view of a furnace demonstrating the
present inventive construction; and
[0016] FIG. 3 is a cross-sectional view of a typical optical
fiber.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In one of its preferred embodiments, the fused quartz
product of the present invention can be formed in a furnace
configuration having the features shown in FIG. 1. The furnace has
a general cylindrical shape. Preferably, an elongated cylindrical
melting crucible 10 constructed of a refractory metal layer 11,
such as tungsten or molybdenum as well as combinations thereof, is
used. The melting crucible 10 further includes a lining of rhenium
13 over the refractory metal layer 11.
[0018] A purified sand raw material is fed through a top opening 12
into a melt zone 14 of the crucible member. The top opening 12 is
provided with movable closure means 16, such as a trapdoor which
can be kept closed except for observing the level of the melt 18
and during feeding of the raw material into the crucible. Automatic
feeder means 20 are provided at the top opening of the crucible
member to maintain a predetermined level of the raw material in the
crucible. The feeder includes a discharge tube 22 having its outlet
opening located in the crucible 10 so as to provide the raw
material in an upper region where melting takes place, a purge gas
inlet tube 24 and reservoir means 26 which contains a supply of the
raw material being fed automatically to the discharge tube.
[0019] The purge gas being supplied to the feeder helps eliminate
gases contained in the raw material which could otherwise form
bubbles in the fused quartz melt which cannot thereafter be removed
or minimized in a manner to be described in part immediately
hereinafter. The composition of the purge gas is generally a gas
mixture of hydrogen and helium in the volume ratios 40-100%
hydrogen and 60-0% helium.
[0020] The lower portion 28 (a drawing zone) of the crucible 10
includes an annular ring 30 having central opening 32 through which
the elongated fused quartz member is continuously formed by drawing
the viscous material through the opening. A core 34 is centrally
disposed in the opening 32 and extends below--but could extend
above--the means of forming tubing from the viscous material being
drawn from the melt. As known by the skilled artisan, the position
of the core can be shifted as necessary to produce the desired size
of extrudate. Support element 35 is affixed to the wall of the
crucible and provides rigid support of the core which helps to
maintain a constant size opening from which the product is being
drawn. The core is fabricated with a hollow interior 36 which is
connected to inlet pipe 38 so that a supply of non-oxidizing gas
can be furnished as a forming atmosphere while the tubing 40 is
being drawn.
[0021] A second inlet pipe 42 supplies what can be a mixture of
hydrogen in a non-oxidizing carrier gas such as argon or nitrogen
in volume ratios 1-20% hydrogen and 99-80% carrier gas as a
protective atmosphere which surrounds the exterior refractory metal
wall 11 of the crucible 10. This supply of gas is provided to
annular space 44 which provides a housing means for the crucible
and includes a central bottom opening 46 providing exhaust means
from the cavity for the gas in a manner which envelops the exterior
surface of the elongated fused quartz member 40 being drawn from
the furnace. The exterior wall of the annular space comprises a
refractory cylinder 48 which in combination with exterior housing
50 of the furnace construction serves as the container means for
the induction heating coils of the apparatus. More particularly, a
concentric passageway 52 is defined between the exterior wall of
the refractory cylinder 48 and the interior wall of housing 50 in
which is disposed two helical-shaped induction heating coils 54 and
56 supplying separate heating sources for the upper and lower zones
of the crucible, respectively. Of course, additional coils may be
employed as governed by the size of the furnace, for example, it
may be beneficial to include additional coil(s) in the finish zone.
In any case, the heating sources and the power supplies thereto can
be of conventional construction.
[0022] A third supply pipe 58 is located in the top section of
exterior housing 50, passing into the crucible 10, allowing a gas
mixture to be fed to the melt zone 14 of the crucible. This gas
mixture is generally an inert carrier gas in combination with an
oxidizing gas. The preferred carrier gas is selected from hydrogen,
helium and the other noble gases and the preferred oxidizing gas is
water vapor or air. Preferably, in the case of hydrogen and water
vapor, the oxidizing gas fed to the melt Zone 14 will be a hydrogen
with a dew point of greater than 30.degree. C., more preferably,
greater than 50.degree..
[0023] The preferred form of the present invention includes the
rhenium lining 13 which enables the introduction of the oxidizing
gas. Moreover, since the refractory metals forming the walls of the
crucible are usually rapidly oxidized and degraded at the
temperature of furnace operation, it is beneficial to protect them
from the oxidizing atmosphere in the melt zone. Of course, any
material suitable to this purpose can be used, such as rhenium,
osmium, iridium and mixtures thereof.
[0024] In prior processes, the presence of hydrogen in the melt
zone to protect the refractory materials also resulted in the Mo/W
oxides being reduced and remaining in the melt as metal particles
causing a loss of strength in the drawn articles. The presence of
oxidizing gas (e.g. water vapor) will keep or convert the
refractory metal oxides to that complexed state, resulting in their
discharge as volatile gases or becoming solubilized into the melt
with little negative impact.
[0025] Of course, the present inventive method and use of a
non-reactive crucible lining in the melt zone is not limited to the
furnace or crucible shown in FIG. 1.
[0026] In accordance with carrying out the process of the present
invention in the above-described apparatus, a natural silica sand
having a nominal particle size of -50 mesh U.S. screen size which
has been purified by chemical treatment to the nominal impurity
content below is supplied to the top opening of the crucible member
in the apparatus. Alternatively, a synthetic silica can be
used.
1 RAW MATERIAL Impurity Natural (p.p.m.) Synthetic (p.p.m.)
Fe.sub.2O.sub.3 1 0.07 TiO.sub.2 2 <.02 Al.sub.2O.sub.3 20 100
CaO 0.4 <.01 MgO 0.1 <.05 K.sub.2O 0.6 0.1 Na.sub.2O 0.7 0.1
Li.sub.2O 0.6 <.05 B <0.2 -- ZrO.sub.2 <1.0 <.02
[0027] The above raw material is provided to the crucible member
which has been heated in excess of 2050.degree. C. while also being
supplied with the hydrogen and helium gas mixture hereinbefore
specified. After a predetermined melt level of fused quartz has
been established in the crucible and the molten material caused to
flow by gravity through central bottom opening 32 in the crucible
member, tubing or rod is then drawn continuously by the drawing
machine (not shown) in the presence of a forming gas atmosphere as
hereinbefore specified. The above-described furnace is operated in
connection with conventional tube or rod drawing machinery which
has been omitted from the drawing as forming no part of the present
invention. In any continuous drawing of tubing/rod in the foregoing
described manner, the electrical power being supplied to the lower
heating coil 56 is typically maintained at a lower level than the
electrical power being supplied to the upper heating coil 54 in
order to lower the temperature of the material as it is being drawn
to below a temperature of 2050.degree. C. However, the use of a
non-reactive lining in the finish zone can allow higher temperature
operation if desired.
[0028] As stated above, the internal surface of the furnace
crucible 10 includes a non-reactive (e.g. rhenium, osmium, platinum
or iridium) sheet or coating 13. The coating 13 may be applied to
the refractory metal layer 11 by chemical vapor deposition,
electrolysis, plasma spray or any other technique known to the
skilled artisan (hereinafter referred to as "chemical bonding").
The non-reactive layer 13 may also be physically attached to the
refractory metal layer 11 by attaching a sheet directly to the wall
of the crucible with rivets, bolts, screws, etc., preferably
constructed from the same or similar material as the non-reactive
lining itself. Alternatively, a properly shaped rhenium sleeve can
be inserted into the crucible. In fact, a combination of coating or
lining methods may be used depending on the geometric complexity of
the segments comprising the crucible assembly.
[0029] Referring now to FIG. 2, an alternative embodiment of the
present invention is demonstrated. Moreover, a sealed cup of
rhenium 113 is located around and above the melt/fusion zone 115.
This position of the cup 113 shields the tungsten walls 117 of the
crucible from the atmosphere 119 in the melt zone 115. This
protection is supplemented by feeding a dry hydrogen gas through
tube 121 to the space 123 between cup 113 and walls 117. A tube 125
is provided to feed wet hydrogen into the melt zone 115, and a tube
126 is provided to exhaust wet hydrogen gas. Of course, proper
seals are provided between tube 125 and sand feed tube 127 to
create a gas barrier within cup 113. As is conventional in the art,
a layer of insulation 129 is disposed between tungsten walls 117
and the induction heating coils 131. As shown in this embodiment,
feed sand 133 is beneficially in a wet hydrogen environment 119 as
it fuses into a molten state 135 for eventual product forming.
[0030] Referring now to FIG. 3, an optical fiber of the present
invention is shown, comprising an optical fiber core 137 surrounded
by a sheath 139 of silica formed via the present inventive
process.
[0031] While the invention has been described by reference to
preferred embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements without departing from the scope of the
invention. In addition, any modifications may be made to adapt a
particular situation or material to the teachings of the invention
without departing from the essential scope thereof. Therefore, it
is intended that the invention not be limited to the particular
embodiment disclosed as the best mode contemplated for carrying out
this invention, but the invention will include all embodiments
falling within the scope of appended claims.
* * * * *