U.S. patent application number 12/871245 was filed with the patent office on 2012-03-01 for method for eliminating carbon contamination of precious metal components.
Invention is credited to Martin H. Goller, David M. Lineman, Steven R. Moshier.
Application Number | 20120047958 12/871245 |
Document ID | / |
Family ID | 45561447 |
Filed Date | 2012-03-01 |
United States Patent
Application |
20120047958 |
Kind Code |
A1 |
Goller; Martin H. ; et
al. |
March 1, 2012 |
METHOD FOR ELIMINATING CARBON CONTAMINATION OF PRECIOUS METAL
COMPONENTS
Abstract
In the formation of sheet material from molten glass, molten
glass is formed in a melting furnace and transported through a
precious metal delivery system to the forming apparatus. Disclosed
herein is a method to eliminate carbon-containing contamination of
individual components of the precious metal delivery system prior
to their installation and use. The method comprises one or more
heat treating steps in an oxygen-containing atmosphere prior to
and/or during assembly of the component.
Inventors: |
Goller; Martin H.; (Painted
Post, NY) ; Lineman; David M.; (Horseheads, NY)
; Moshier; Steven R.; (Horseheads, NY) |
Family ID: |
45561447 |
Appl. No.: |
12/871245 |
Filed: |
August 30, 2010 |
Current U.S.
Class: |
65/374.12 ;
148/678 |
Current CPC
Class: |
C22B 9/14 20130101; C03B
5/187 20130101 |
Class at
Publication: |
65/374.12 ;
148/678 |
International
Class: |
C22F 1/14 20060101
C22F001/14; C03B 8/00 20060101 C03B008/00 |
Claims
1. A method of eliminating carbon from a platinum-containing
metallic component for use in a glass making system comprising:
providing a first platinum-containing metallic member comprising a
concentration of carbon greater than 3 ppm; and heating the first
platinum-containing metallic member in a first heat treating step
at a temperature and for a period of time effective to reduce the
concentration of carbon to less than 3 ppm.
2. The method according to claim 1, wherein the first
platinum-containing metallic member is heated to a temperature
.gtoreq.1200.degree. C. for a period of time .gtoreq.12 hours in an
atmosphere containing .gtoreq.20% by volume oxygen.
3. The method according to claim 2, wherein the first
platinum-containing member is heated to a temperature
.gtoreq.1450.degree. C.
4. The method according to claim 2, wherein the first
platinum-containing member is heated for a period of time
.gtoreq.72 hours.
5. The method according to claim 1, further comprising coupling
together the first platinum-containing metallic member and a second
platinum-containing metallic member in an overlapping relationship
to form an assembly with a first interstitial space formed between
the first and second platinum-containing metallic members.
6. The method according to claim 5, further comprising heating the
assembly in a second heat treating step to a temperature of at
least 1200.degree. C. for a period of time .gtoreq.12 hours in an
atmosphere containing .gtoreq.20% by volume oxygen.
7. The method according to claim 1, further comprising coupling
together the first platinum-containing metallic member and a
plurality of subsequent platinum-containing metallic members, and
heat treating the first platinum-containing metallic member after
the coupling of each subsequent platinum-containing metallic member
at a temperature .gtoreq.1200.degree. C. for a period of time
.gtoreq.12 hours in an atmosphere containing .gtoreq.20% by volume
oxygen.
8. The method according to claim 1, wherein the platinum-containing
metallic component comprises a vessel, the method further
comprising flowing molten glass through the vessel.
9. A method of making a platinum-containing component for use in a
glass making system comprising: a. providing a first
platinum-containing metallic member and a second
platinum-containing member; b. coupling together the first
platinum-containing member and the second platinum-containing
member in an overlapping relationship to form an assembly with a
first interstitial space formed between the first and second
platinum-containing members, wherein the assembly comprises carbon
in a concentration greater than 3 ppm; and c. heating the assembly
in a first heat treating step at a temperature and for a period of
time effective to reduce the concentration of carbon to less than 3
ppm.
10. The method according to claim 9, wherein the assembly is heated
to a temperature .gtoreq.1200.degree. C. for a period of time
.gtoreq.12 hours in an atmosphere containing .gtoreq.20% by volume
oxygen.
11. The method according to claim 9, wherein the assembly is heated
to a temperature .gtoreq.1450.degree. C. during the first heat
treating step.
12. The method according to claim 9, wherein the assembly is heated
for a period of time 72 hours.
13. The method according to claim 9, further comprising coupling at
least one additional platinum-containing member to the first or the
second platinum-containing member after the first heating treating
step, there being a second interstitial space formed between the at
least one additional platinum-containing member and the first or
second platinum-containing member, and repeating step c.
14. The method according to claim 9, wherein the assembly comprises
a hollow tube comprising a plurality of nested platinum-containing
members.
15. The method according to claim 9, wherein the atmosphere
contains at least 30% oxygen.
16. The method according to claim 9, wherein the assembly is a
sub-assembly of a molten glass stirring apparatus.
17. The method according to claim 9, further comprising lubricating
the first platinum-containing member or the second
platinum-containing member with a carbon-containing material prior
to step b.
18. A method of making a stirrer for stirring molten glass
comprising: providing a first platinum-containing metallic member;
contacting the first platinum-containing member with a
carbon-containing material; and heating the first
platinum-containing metallic member subsequent to the contacting in
a first heat treating step to a temperature .gtoreq.1200.degree. C.
for a period of time .gtoreq.12 hours in an atmosphere containing
.gtoreq.20% by volume oxygen, and wherein the first
platinum-containing member comprises carbon in a concentration less
than 3 ppm after the heating.
19. The method according to claim 18, further comprising coupling
together the first platinum-containing metallic member and a
plurality of subsequent platinum-containing metallic members, and
heat treating the first platinum-containing metallic member after
the coupling of each subsequent platinum-containing metallic member
at a temperature .gtoreq.1200.degree. C. for a period of time
.gtoreq.12 hours in an atmosphere containing .gtoreq.20% by volume
oxygen.
20. The method according to claim 18, wherein the first
platinum-containing metallic member is heated to a temperature
.gtoreq.1450.degree. C. during the first heat treating step.
Description
FIELD
[0001] This invention relates to a method for joining precious
metal components, and more particularly for reducing gaseous
blisters in molten glass that originate from carbon contamination
of precious metal components of a glass making system.
BACKGROUND
[0002] Glass making system for delivering high quality glass in the
manufacture of precision glass articles requires careful attention
to the delivery systems. Such precision products can include
optical lenses and glass panels for the manufacture of display
devices.
[0003] The molten glass delivery systems for high precision
products may typically be formed from precious metals, and usually
platinum or platinum alloys such as a platinum rhodium alloy. Such
precious metals, usually selected from the platinum group of
metals, have high melting temperatures, and are less likely to
contribute contaminants to the molten glass (melt) flowing through
these "platinum" delivery systems. In many instances, individual
components of a particular platinum delivery system, a finer for
example, or a stirring vessel, are produced by joining multiple
subcomponents. For example, a cylindrical tube might be formed by
rolling several flat platinum plates into semicircular segments,
then welding the segments to form the tube. In another example,
stirrers for stirring the molten glass may be formed by welding
individual stirring blades to a shaft. Even the shaft may be formed
from multiple components.
[0004] In spite of the relatively benign behavior of platinum (or
platinum alloy) when submerged within the corrosive molten glass,
it has been found that some of these platinum components may be
contributing to inadvertent contamination of the molten glass with
gaseous inclusions, or blisters.
[0005] Blisters believed to originate from precious metal
components, such as an apparatus for stirring molten glass, have
been identified as a significant loss issue in the manufacture of
glass sheet for LCD display substrates. The problem is especially
prevalent during startup of a melting furnace, but has also been
observed mid-campaign. Because the defects constitute greater than
about 90% CO.sub.2, the underlying problem is believed to be carbon
contamination of the components. The carbon contamination may be
present in the components as-received from the component
manufacturer, or it might be introduced into the component during
operation.
[0006] The following disclosure addresses method of treating
individual components and/or sub-components prior to and during
assembly to mitigate the formation of these gaseous inclusions.
SUMMARY
[0007] Disclosed herein are embodiments of methods to produce
platinum-containing articles for use in a glass making system
comprising no, or very small amounts (less than about 3 ppm) of
carbon. Carbon content in an amount greater than about 3 ppm, and
in some cases greater than 2 ppm, may result in the formation of
CO.sub.2 gas at the interface between the platinum and molten glass
that produces bubbles in the molten material that persist,
undesirably, into the final glass article. The carbon can come from
a variety of sources, but most commonly occurs when carbon
containing lubricants are used in the manufacture of
platinum-containing sub-assemblies and assemblies.
Platinum-containing components are often used in delivery systems
for transporting the molten material from one location to another,
or for processing the molten mass, such as homogenizing the
material due to the high temperature resistant capabilities of the
metal. Such articles may be formed from platinum, or a platinum
alloy, such as, but not limited to platinum-rhodium alloys and
platinum-iridium alloys. Conventional cleaning methods, such as
washing with detergents, may not remove carbon that diffuses into
the body of the platinum article. Therefore, other methods may be
required to eliminate the carbon.
[0008] The molten glass may also be referred to as the glass melt
or simply melt. It should be understood that glass as commonly
understood comprises an elastic state, and that although the molten
material produced by the melter is not at that point truly a glass,
it is capable of forming a glass upon cooling, and those skilled in
the art of glass making will understand the reference.
[0009] In accordance with one embodiment, a method of making a
platinum-containing metallic component for use in a glass making
system is described comprising providing a first
platinum-containing metallic member and heating the first
platinum-containing metallic member in a first heat treating step
to a temperature of .gtoreq.1200.degree. C., .gtoreq.1450.degree.
C., .gtoreq.1600.degree. C. and in some instances
.gtoreq.1650.degree. C. in an atmosphere containing .gtoreq.20% by
volume oxygen. According to the present method, the component
should be heat treated at the prescribed temperature for a period
of time .gtoreq.12 hours, .gtoreq.24 hours, .gtoreq.36 hours,
.gtoreq.48 hours and in some instances .gtoreq.72 hours. For
reduced heat treatment temperatures, for example in a temperature
range .gtoreq.1200.degree. C. but less than 1450.degree. C., the
heat treatment may be continued for longer periods of time than at
higher temperatures. For example, the heat treatment can be
continued for more than 12 hours if the heat treatment temperature
is in the range between from .gtoreq.1200.degree. C. but less than
1450.degree. C. The time and temperature can be selected based on
such factors as the expected level of carbon contamination and
thickness of the platinum-containing parts.
[0010] If additional layers are to be formed, the method may
further comprise coupling together the first platinum-containing
metallic member and a second platinum-containing metallic member in
an overlapping relationship to form an assembly with a first
interstitial space formed between the first and second
platinum-containing metallic members. This overlapping relationship
may include overlapping one surface over another surface, such as
one broad surface area of one the first platinum-containing member
overlapping the broad surface area of another platinum-containing
member. However, it may also include the simple attachment of one
component to another component, even in an edge-wise fashion, for
example, the attachment of a blade to a stirrer shaft.
[0011] In the instance where the first platinum-containing member
is coupled to another platinum-containing member, the assembly
resulting from the coupling can be heated treated after each such
coupling. Each subsequent heat treating step includes heating the
assembly to a temperature of at least 1200.degree. C. for a period
of time .gtoreq.12 hours in an atmosphere containing .gtoreq.20% by
volume oxygen. However, the temperature may be selected to be
.gtoreq.1200.degree. C., .gtoreq.1450.degree. C.,
.gtoreq.1600.degree. C. and in some instances .gtoreq.1650.degree.
C. The heat treatment may be continued for a period of time
.gtoreq.12 hours, .gtoreq.24 hours, .gtoreq.36 hours, .gtoreq.48
hours and in some instances .gtoreq.72 hours.
[0012] The method may further comprise coupling together the first
platinum-containing metallic member and a plurality of subsequent
platinum-containing metallic members, and, as in the preceding
paragraph, heat treating the first platinum-containing metallic
member after the coupling of each subsequent platinum-containing
metallic member at a temperature 1200.degree. C. for a period of
time .gtoreq.12 hours in an atmosphere containing .gtoreq.20% by
volume oxygen. However, the temperature may be selected to be
.gtoreq.1200.degree. C., .gtoreq.1450.degree. C.,
.gtoreq.1600.degree. C. and in some instances .gtoreq.1650.degree.
C. The heat treatment may be continued for a period of time
.gtoreq.12 hours, .gtoreq.24 hours, .gtoreq.36 hours, .gtoreq.48
hours and in some instances 72 hours.
[0013] In another embodiment, a method of making a
platinum-containing component for use in a glass making system is
disclosed comprising providing a first platinum-containing metallic
member and a second platinum-containing member, coupling together
the first platinum-containing member and the second
platinum-containing member in an overlapping relationship to form
an assembly with a first interstitial space formed between the
first and second platinum-containing members and heating the
assembly in a first heat treating step to a temperature of at least
1200.degree. C. for a period of time .gtoreq.12 hours in an
atmosphere containing .gtoreq.20% by volume oxygen. However, the
heat treatment temperature may be selected to be
.gtoreq.1200.degree. C., .gtoreq.1450.degree. C.,
.gtoreq.1600.degree. C. or .gtoreq.1650.degree. C. The heat
treatment may be continued for a period of time .gtoreq.12 hours,
.gtoreq.24 hours, .gtoreq.36 hours, .gtoreq.48 hours and in some
instances .gtoreq.72 hours. The atmosphere may contain by volume
.gtoreq.30% oxygen, .gtoreq.40% oxygen, .gtoreq.50% oxygen,
.gtoreq.60% oxygen, .gtoreq.70% oxygen, .gtoreq.80% oxygen,
.gtoreq.90% oxygen or even 100% oxygen.
[0014] The method may further comprise coupling at least one
additional platinum-containing member to the first or the second
platinum-containing member after the first heating treating step,
with a second interstitial space formed between the at least one
additional platinum-containing member and the first or second
platinum-containing member, and repeating the heat treating step.
The assembly may, for example, comprise a hollow tube including a
plurality of nested platinum-containing members.
[0015] The assembly may be a sub-assembly of a molten glass
stirring apparatus. Alternatively, the assembly may be any
component or sub-component of a platinum-containing article that
contacts molten glass.
[0016] The method may further comprise lubricating either or both
of the first platinum-containing member or the second
platinum-containing member prior to or during the step of coupling
the platinum-containing members together. For example, a lubricant
is often used in the process of manufacturing and/or assembling
individual components.
[0017] In still another embodiment, a method of eliminating carbon
from a platinum-containing metallic member is disclosed comprising
providing a first platinum-containing metallic member; contacting
the first platinum-containing member or the second
platinum-containing member with a carbon-containing material and
heating the first platinum-containing metallic member subsequent to
the contacting in a first heat treating step to a temperature of at
least 1200.degree. C. for a period of time .gtoreq.12 hours in an
atmosphere containing .gtoreq.20% by volume oxygen to eliminate
dissolved carbon from an interior portion of the first
platinum-containing metallic member. However, the temperature may
be selected to be .gtoreq.1200.degree. C., .gtoreq.1450.degree. C.,
.gtoreq.1600.degree. C. or .gtoreq.1650.degree. C. The heat
treatment may be continued for a period of time .gtoreq.12 hours,
.gtoreq.24 hours, .gtoreq.36 hours, .gtoreq.48 hours or 72 hours.
The atmosphere may contain by volume .gtoreq.30% oxygen,
.gtoreq.40% oxygen, .gtoreq.50% oxygen, .gtoreq.60% oxygen,
.gtoreq.70% oxygen, .gtoreq.80% oxygen, .gtoreq.90% oxygen or even
100% oxygen.
[0018] The method may further comprise coupling together the first
platinum-containing metallic member and a plurality of subsequent
platinum-containing metallic members, and heat treating the first
platinum-containing metallic member after the coupling of each
subsequent platinum-containing metallic member at a temperature of
at least 1450.degree. C. for a period of time .gtoreq.12 hours in
an atmosphere containing .gtoreq.20% by volume oxygen. However, the
heat treatment temperature may be selected to be
.gtoreq.1200.degree. C., .gtoreq.1450.degree. C.,
.gtoreq.1600.degree. C. or .gtoreq.1650.degree. C. The heat
treatment may be continued for a period of time .gtoreq.12 hours,
.gtoreq.24 hours, .gtoreq.36 hours, .gtoreq.48 hours and in some
instances .gtoreq.72 hours. The atmosphere may contain by volume
.gtoreq.30% oxygen, .gtoreq.40% oxygen, .gtoreq.50% oxygen,
.gtoreq.60% oxygen, .gtoreq.70% oxygen, .gtoreq.80% oxygen,
.gtoreq.90% oxygen or even 100% oxygen.
[0019] Additional features and advantages of the invention are set
forth in the detailed description which follows, and in part will
be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein. The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification. It is to be understood
that the various features of the invention disclosed in this
specification and in the drawings can be used in any and all
combinations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is an elevational view in partial cross section
showing an exemplary fusion downdraw process for the manufacture of
glass sheet, and showing the platinum delivery system for
transporting molten glass from the melting furnace to the forming
body;
[0021] FIG. 2 is a cross sectional view of a stirring apparatus for
homogenizing the molten glass as it flows through the platinum
delivery system;
[0022] FIG. 3 is a close up cross sectional view of a portion of
the shaft of a stirrer disposed in the stirring apparatus of FIG.
2;
[0023] FIG. 4 is a close up cross sectional view of the portion of
the shaft of FIG. 3, showing the several layers of precious metal
comprising the stirrer shaft;
[0024] FIG. 5 is a graph showing the solubility of carbon in
platinum as a function of temperature;
[0025] FIG. 6 is a graph showing the minimum concentration of
carbon needed to achieve a partial pressure of CO.sub.2 (pCO.sub.2)
equal to 1 atmosphere as a function of the partial pressure of
oxygen (pO.sub.2) in the platinum.
[0026] FIG. 7 is a graph indicating the length of heat treating
time needed to remove carbon at an initial concentration of 1.1% to
a concentration of 3 ppm as a function of metal thickness for a
range of temperatures between 1100.degree. C. and 1500.degree.
C.
[0027] FIG. 8 depicts a non-heat treated platinum-rhodium alloy
sealed pouch containing 10 mg of carbon that was immersed in a bath
of molten glass for 24 hours at 1450 C and shows abundant blister
formation.
[0028] FIG. 9 depicts a platinum-rhodium alloy sealed pouch
containing 10 mg of carbon that was heat treated for 168 hours at
1450.degree. C., then immersed in a bath of molten glass for 24
hours at 1450.degree. C., and shows a significant decrease in
blister formation when compared to the sample of FIG. 8.
DETAILED DESCRIPTION
[0029] In the following detailed description, for purposes of
explanation and not limitation, example embodiments disclosing
specific details are set forth to provide a thorough understanding
of the present invention. However, it will be apparent to one
having ordinary skill in the art, having had the benefit of the
present disclosure, that the present invention may be practiced in
other embodiments that depart from the specific details disclosed
herein. Moreover, descriptions of well-known devices, methods and
materials may be omitted so as not to obscure the description of
the present invention. Finally, wherever applicable, like reference
numerals refer to like elements.
[0030] Shown in FIG. 1 is a side view of an exemplary glass making
apparatus 10 comprising melting furnace or melter 12, finer 14,
stirring apparatus 16, collection vessel 18, and downcomer tube 20
for supplying molten glass to a forming body 22 for producing a
thin ribbon of glass. Finer 14 is connected to melter 12 through
melter to finer connecting tube 24 and to stirring apparatus 16
through connecting tube 26. Collection vessel 18 is connected
upstream to stirring apparatus 16 through connecting tube 28.
Downcomer tube 20 is connected to collection vessel 18, and
supplies molten glass to inlet 30 connected to forming body 22.
Melter 12 is typically constructed from a refractory material, such
as alumina or zirconia, and is supplied with batch material that is
melted by, for example, a gas flame and/or an electric current
passed between electrodes in the melter structure. Similarly,
forming body 22 is also typically formed from a refractory
material. In this instance, glass making apparatus 10 comprises a
fusion downdraw system, so named because molten glass delivered to
the forming body overflows both sides of the forming body as
separate flows, then re-join or fuse at the bottom of the forming
body as the molten glass is drawn downward by pulling rollers to
produce a thin, pristine ribbon of glass 31. The ribbon may be cut
at the bottom of the draw area into individual glass sheets. It
should be noted, however, that the forming process itself may be
replaced with just about any other forming process, as it is the
delivery system, i.e. those precious metal components between the
melter and the forming body, that are the subject of the present
disclosure. These components include finer 14, stirring apparatus
16, collection vessel 18, downcomer tube 20, inlet 30 and
connecting tubes 24, 26, and 28, and are collectively referred to
herein as the platinum system, so-called because each of the
components is formed from platinum or a platinum alloy metal such
as a platinum rhodium alloy, or coated or clad with platinum or a
platinum alloy. Moreover, while the present disclosure is presented
in the context of the exemplary platinum system introduced above,
the principals and teaching of the present disclosure is applicable
any time platinum components are assembled for use in a glass
making system. In addition, the present invention is not limited to
a fusion glass making system, but may be applied to other glass
making processes.
[0031] According to the exemplary fusion glass making system above,
raw, batch materials 32, are sourced to the melting furnace (as
indicated by arrow 34) where heat is applied to melt the individual
constituents of the batch and form the molten glass 36. The batch
materials typically include various metal oxides and other
additives as required for a specific glass composition. The melter
itself is typically formed from a refractory material, for example
refractory bricks. The melting process produces, inter alia,
various gases that are entrained into the molten glass and must be
removed if a quality product is to be produced from the molten
mixture. Thus, a fining step is included. For example, the molten
glass can be flowed by gravity from melter 12 through connecting
tube 24 to finer 14, where the temperature of the molten glass is
raised. The increased temperature both decreases the viscosity of
the molten glass, and causes certain fining agents (e.g.
multivalent compounds such as arsenic oxide, tin oxide and/or
antimony oxide) included in the batch material to release gas, e.g.
oxygen bubbles. The gas released by the fining agent enters
existing bubbles, causing them to grow and therefore rise through
the glass melt faster. The increased temperature also results in a
decrease in the viscosity of the molten glass that allows the
bubbles to rise faster. Fining is achieved when the bubbles rise to
a free surface of the molten glass and escape from the melt.
[0032] Once the molten glass has been fined, the molten glass is
flowed through connecting tube 26 to stirring apparatus 16
comprising stirring vessel 38, stirrer 40 rotatably disposed in the
stirring vessel. Molten glass flows into the stirring vessel 38
through stirring vessel inlet 42 and is stirred by stirrer 40.
Stirrer 40 typically includes stirrer shaft 44 coupled to motor 46
through a drive mechanism (e.g., chain 48 and sprockets 50) and
coupler 52. Stirrer 40 also includes blades 54 arranged on the
shaft such that the blades are submerged in the molten glass during
operation of the stirrer. Stirrer 40 homogenizes the molten glass,
and removes and/or dissipates cord and other anomalies typically
resulting from refractive index differences originating from
compositional inhomogeneities. From stirring apparatus 16 the
molten glass flows from stirring vessel outlet 56 through
connecting tube 28 to collection vessel 18, and then through
downcomer tube 20 to inlet 30 of forming body 22.
[0033] Each of the components of the platinum delivery system
described above may be formed from smaller sub-components, and
assembled, such as by welding. The following description will
review assembly of stirrer 40, but it should be understood that the
following principals can be applied to other components of the
platinum system and are not limited to the stirrer or stirring
apparatus.
[0034] FIG. 2 depicts a portion of shaft 44 comprising stirrer 40.
As shown, shaft 44 is a hollow cylinder comprised of multiple
layers of a platinum-containing metal that form the wall of the
hollow cylinder. For example, the platinum-containing metal may be
a platinum rhodium (Pt--Rh) alloy, such as 80% platinum and 20%
rhodium. While two layers are shown, inner layer 58 and outer layer
60, shaft 44 may comprise more layers, as needed. Between the inner
layer 58 and outer layer 60 is an interstitial space 62. During
assembly of the shaft, the individual layers of the shaft are
nested. Such nesting provides for increased strength, while
allowing for the use of a hollow shaft that reduces the amount of
expensive platinum. However, inner layer 58 and outer layer 60 are
not substantially intimately joined. By not being substantially
intimately joined what is meant is that the layers are primarily
discrete, and not substantially joined by melting that would
co-mingle the layers for example. The interstitial space may be
only a few molecules thick in some instances, and need not be
continuous. For example, shaft 44 can be formed by press fitting a
first platinum-containing metal tube into a second
platinum-containing metal tube, or by co-extrusion, which may leave
a small interstitial space between the tubes. However, portions of
the layers may be intimately joined such as by welding along edges
of the layers (such as the top or bottom of the shaft) to ensure
the integrity of the assembly, e.g. the shaft.
[0035] During manufacture of the individual layers of the shaft,
various organic (carbon-containing) lubricants may be employed. For
example, lubricants are routinely used during extrusion, rolling or
pressing operations. Carbonaceous (carbon-containing) material
comprising the lubricant can be trapped in the stirrer structure
between the various layers of platinum-containing metal of the
structure. In addition, carbon can end up being cold-worked into
the article during its fabrication. At an operating temperature of
the stirring apparatus, the carbon dissolves in the
platinum-containing metal and diffuses through the stirrer shaft
wall toward the hollow interior space 64 of the stirrer shaft, as
indicated by arrows 66, and toward the melt, as indicated by arrows
68. Carbon that may interact with oxygen within the interior space
64 to form CO or CO.sub.2 gas is released through vent 70 above the
molten glass. Vent 70 also allows oxygen into the interior space
where it can further react with the carbon. As best seen in FIG. 4,
carbon diffusing in the opposite direction encounters oxygen from
the glass melt stirrer-melt interface and reacts to form CO or
CO.sub.2 gas at the outer surface 72 of outer layer 60, and
subsequently produces bubbles 74 that become entrained within the
molten glass.
[0036] As shown in FIG. 5 depicting the solubility of carbon in
platinum, carbon is quite soluble in Pt at stirrer operating
temperatures (e.g. less than about 1500.degree. C., depending on
the glass composition), and with no intermediate phases. In
addition, the diffusivity of carbon in Pt is reasonably high
(.about.10.sup.-5 cm.sup.2/s) at stirrer operating temperatures.
For this mechanism to be responsible for observed CO.sub.2
blisters, the flux of carbon through the Pt wall must be at least
as high as the flux of carbon in the observed CO.sub.2 blisters.
For example, the concentration of carbon on interior surface 76 of
stirrer shaft 44 needed to produce blisters has been calculated in
some instances to be approximately 2 ppm, a level quite easily
achieved.
[0037] There is also a thermo-chemical criterion that must be met
for carbon diffusion to be responsible for observed CO.sub.2
blisters. To nucleate a CO.sub.2 bubble, the partial pressure of
CO.sub.2 (pCO.sub.2) must be greater than about 1 atmosphere. FIG.
6 shows the minimum concentration in mol fraction of carbon in Pt
needed to achieve pCO.sub.2=1 atm and nucleate a bubble as a
function of the partial pressure of oxygen in the glass melt
(pO.sub.2) at 1425.degree. C., a typical stirring apparatus
operating temperature for display-type glass. It should be noted
the 2.times.10.sup.-13 value is strictly based on equilibrium
conditions, whereas the 2 ppm value previously described is based
on a kinetic or reaction rate.
[0038] To eliminate carbonaceous material from platinum-containing
sub-components of the platinum system, one or more heat treating
processes are applied to individual sub-assemblies. Again using the
stirrer shaft as an example, once the two-layer structure
comprising inner layer 58 and outer layer 60 has been assembled,
such as by pressing a first platinum-containing hollow metal
cylinder into a second platinum-containing hollow metal cylinder,
the outside surfaces of the resulting first sub-assembly can be
easily cleaned with a suitable solvent. However, the interstitial
space (interface) between the first and second metal cylinders is
not so easily cleaned, and may not even be accessible, and any
lubricant used in forming and/or assembling the layer structure
that becomes disposed between the inner layer and the outer layer
is virtually impossible to remove via conventional cleaning or
washing methods.
[0039] To ensure the removal of any residual carbonaceous material,
the first sub-assembly (e.g. nested inner layer 58 and outer layer
60) is heat treated by heating the sub-assembly to a temperature of
at least 1200.degree. C. for a period of time equal to or greater
than 12 hours in an atmosphere containing equal to or greater than
20% by volume oxygen. The atmosphere may be air. Alternatively, the
atmosphere may contain by volume .gtoreq.30% oxygen, .gtoreq.40%
oxygen, .gtoreq.50% oxygen, .gtoreq.60% oxygen, .gtoreq.70% oxygen,
.gtoreq.80% oxygen, .gtoreq.90% oxygen or even 100% oxygen. In some
embodiments, the temperature can be as high as 1450.degree. C.,
1600.degree. C. or even 1650.degree. C. However, care should be
taken not to cause oxidation damage to the sub-assembly, so the
temperature and oxygen content should be appropriately balanced.
For heat treatment steps conducted at lower temperatures, such as
.gtoreq.1200.degree. C., the time period may be longer than the
time period under high temperature conditions. The selected time
period for the heat treatment, based among other things on the
thicknesses of the layers and the expected level of carbon
contamination, can range from .gtoreq.12 hours, .gtoreq.24 hours,
.gtoreq.36 hours, .gtoreq.48 hours and in some instances .gtoreq.72
hours.
[0040] In addition, the thickness of the platinum-containing
sub-assembly, e.g. a thickness of a wall of the sub-assembly,
should be considered. For example, FIG. 7 depicts the length of
heat treating time to remove carbon from metal at an initial amount
of 1.1% to a level of 3 ppm as a function of metal thickness for a
range of temperatures between 1100.degree. C. and 1500.degree. C.,
i.e. 1100.degree. C. (curve 78), 1200.degree. C. (curve 80),
1300.degree. C. (curve 82), 1400.degree. C. (curve 84) and
1500.degree. C. (curve 86). These curves are based on diffusion of
carbon out of a metal slab and can be applied to each of the
embodiments disclosed herein. It should be noted that a carbon
concentration less than 1.1% would require less time for heat
treating as that indicated.
[0041] Assuming an initial carbon concentration of 1.1% in
platinum, which is near the saturation limit, the time needed to
reach an average concentration of 3 ppm can be calculated using
equation (1) below:
M t M 0 = n = 0 .infin. 8 ( 2 n + 1 ) 2 .pi. 2 exp ( - D ( 2 n + 1
) 2 .pi. 2 t / 4 L 2 ) ( 1 ) ##EQU00001##
where M.sub.0 is the initial amount of carbon in the platinum (e.g.
1.1%) and M.sub.t is the amount of carbon at a given time and D is
the diffusion coefficient for carbon in Pt, in the range from about
10.sup.-7 to 10.sup.-5 cm.sup.2/sec from 1100.degree. C. to
1500.degree. C. Substituting 3 ppm (3.times.10.sup.-4%) for
M.sub.t, equation (1) can be solved for the time t to get to 3
ppm.
[0042] If additional layers are to be added, the heat treating step
above can be repeated for each additional layer, and/or a final
heat treating step applied after the final layer has been added.
Each heat treating step need not be the same
time-temperature-oxygen content as a preceding heat treating step.
The number of heat treating steps, and the time-temperature-oxygen
content of each heat treating step, will depend on the particulars
of the construction of the component being assembled.
[0043] Because carbon is reasonably soluble in platinum, even
single layer platinum-containing metal articles can become
contaminated with carbon during processing of the article. The
carbon can diffuse into the platinum-containing article, where it
is virtually impossible to remove by conventional cleaning
solutions. It should be apparent, therefore, that, based on the
preceding, the heat treating process need not be confined to the
described multi-layer assembly, but can be applied to single layer
platinum-containing metal articles prior to their assembly into
more complex pieces, or, in the instance where the single-layer
article is used as is, prior to its use in the glass making system.
For example, a platinum-containing metal tube comprising only a
single layer of platinum-containing metal can be heat treated by
heating the single layer article to a temperature of
.gtoreq.1200.degree. C., .gtoreq.1450.degree. C.,
.gtoreq.1600.degree. C. or .gtoreq.1650.degree. C. for a period of
time equal to or greater than 12 hours in an atmosphere containing
equal to or greater than 20% by volume oxygen. The atmosphere may
be air. Alternatively, the atmosphere may contain by volume
.gtoreq.30% oxygen, .gtoreq.40% oxygen, .gtoreq.50% oxygen,
.gtoreq.60% oxygen, .gtoreq.70% oxygen, .gtoreq.80% oxygen,
.gtoreq.90% oxygen or even 100% oxygen. In some embodiments, the
temperature can be as high as 1600.degree. C. or even 1650.degree.
C. However, care should be taken not to cause oxidation damage to
the sub-assembly, so the temperature and oxygen content should be
appropriately balanced.
Example
[0044] A test sample (Sample 1) in the form of a pouch was produced
from two 0.762 mm thick sheets of 90% Pt-10% Rh alloy. A 10 mg
sample of carbon was sealed inside the pouch by welding the edges
of the sheets. The sample was then immersed in a bath of molten
glass (Eagle XG.TM. glass manufactured by Corning Incorporated) at
1450.degree. C. for 24 hours. Sample 1 (FIG. 8) had no prior heat
treatment of the platinum-rhodium metal according to the
embodiments described herein prior to the immersion. FIG. 8
illustrates abundant generation of blisters (bubbles) at the
surface of the sealed metal pouch resulting from the high
temperature immersion.
[0045] For comparison, another sample (Sample 2) was produced in a
manner identical to the manner in which Sample 1 was made, and also
had 10 mg of carbon disposed within the sealed pouch. Sample 2 was
heat treated at 1450.degree. C. for 7 days (168 hours) prior to
immersing the sample in a bath of the molten Eagle XG.TM. glass,
also at 1450.degree. C., according to Sample 1. FIG. 9 shows Sample
2 after a 24 hour immersion in the molten glass at a temperature of
1450.degree. C. Sample 2 shows a dramatic reduction in blister
formation from the heat treated metal when compared with the
blistering apparent from the non-heat treated metal (Sample 1
above).
[0046] It should be emphasized that the above-described embodiments
of the present invention are merely possible examples of
implementations set forth for a clear understanding of the
principles of the invention. Many variations and modifications may
be made to the above-described embodiments of the invention without
departing substantially from the spirit and principles of the
invention. For example, while the above description has been
presented in terms of a stirrer shaft, the principals described can
be applied to other single or multi-layer platinum-containing
components of a glass making apparatus that come into contact with
molten glass, including but not limited to single or double-walled
tubes or pipes used to transport the molten glass from one location
to another location, vessels for conditioning the molten glass, and
sub-assemblies of certain components, such as stirrer blades
coupled or uncoupled to the stirrer shaft. All such modifications
and variations are intended to be included herein within the scope
of this disclosure and the present invention and protected by the
following claims.
* * * * *