U.S. patent application number 10/929819 was filed with the patent office on 2006-03-02 for low melting temperature silver braze alloy.
Invention is credited to Gregory W. Alexander, Karl J. JR. Haltiner, Gary F. Reisdorf.
Application Number | 20060045791 10/929819 |
Document ID | / |
Family ID | 35241154 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060045791 |
Kind Code |
A1 |
Haltiner; Karl J. JR. ; et
al. |
March 2, 2006 |
Low melting temperature silver braze alloy
Abstract
A brazing alloy comprising elemental silver alloyed with
elemental silicon in a silver/silicon ratio between about 95/5 and
99/1, preferably about 97/3. Small amounts of silicon alloyed with
silver depress the alloy liquidus curve significantly, the liquidus
temperature of a silver and silicon eutectic alloy being about
837.degree. C. Brazing alloys in accordance with the invention are
useful in bonding ceramics to ceramics, ceramics to metals, and
metals to metals. Copper, vanadium, or other oxygen-reactive
surface bonding elements may also be included. Silver/silicon
alloys are useful in applications such as assembly of components of
solid oxide fuel cells. A variety of silver/silicon alloy brazes
can be used within the same fuel cell so that subsequent brazing
can be performed without reliquifying a previous braze. A brazing
alloy comprising elemental silver and ruthenium in a
silver/ruthenium ratio between 97/3 and 99/1 is also included.
Inventors: |
Haltiner; Karl J. JR.;
(Fairport, NY) ; Alexander; Gregory W.;
(Pittsford, NY) ; Reisdorf; Gary F.; (Penfield,
NY) |
Correspondence
Address: |
Paul L. Marshall, Esq.;Delphi Technolgies, Inc.
Mail Code 480410202
P.O. Box 5052
Troy
MI
48007
US
|
Family ID: |
35241154 |
Appl. No.: |
10/929819 |
Filed: |
August 30, 2004 |
Current U.S.
Class: |
420/501 ;
420/505 |
Current CPC
Class: |
B23K 35/322 20130101;
B23K 35/32 20130101; H01M 8/0282 20130101; B23K 35/30 20130101;
B23K 35/3006 20130101; B23K 35/0222 20130101; C04B 2237/125
20130101; B23K 35/0244 20130101; Y02E 60/50 20130101; H01M
2008/1293 20130101; C04B 37/026 20130101; C04B 37/006 20130101 |
Class at
Publication: |
420/501 ;
420/505 |
International
Class: |
C22C 5/06 20060101
C22C005/06 |
Goverment Interests
[0001] This invention was made with United States Government
support under Government Contract/Purchase Order No.
DE-FC26-02NT41246. The Government has certain rights in this
invention
Claims
1-8. (canceled)
9. A fuel cell assembly comprising a first component and a second
component wherein said first and second components are bonded by a
brazing alloy consisting essentially of silver and silicon, wherein
said alloy has a liquidus temperature of less than 962.degree.
C.
10. A fuel cell assembly in accordance with claim 9 wherein the
weight percent ratio of said silver to said silicon is between
about 99/1 and about 95/5.
11. A fuel cell assembly in accordance with claim 23 wherein the
weight percent ratio of said silver to said ruthenium is between
about 99/1 and about 97/3.
12. A fuel cell assembly in accordance with claim 9 wherein at
least one of said first and second components is a ceramic
component.
13. A fuel cell assembly in accordance with claim 9 wherein at
least one of said first and second components is a metal
component.
14. A fuel cell assembly in accordance with claim 9 wherein said
brazing alloy is a first brazing alloy, said fuel assembly further
comprising an additional element wherein said additional element is
bonded to one of said first element, second element and another
element by a second brazing alloy having a liquidus temperature
less than said liquidus temperature of said first brazing
compound.
15. A fuel cell assembly in accordance with claim 9 wherein said
assembly comprises a solid oxide fuel cell.
16-18. (canceled)
19-21. (not entered)
22. A fuel cell assembly in accordance with claim 10 wherein said
liquidus temperature is about 835.degree. C.
23. A fuel cell assembly comprising a first component and a second
component wherein said first and second components are bonded by a
brazing alloy consisting essentially of silver and ruthenium,
wherein said alloy has a liquidus temperature of less than
962.degree. C.
24. A fuel cell assembly in accordance with claim 23 wherein at
least one of said first and second components is a ceramic
component.
25. A fuel cell assembly in accordance with claim 23 wherein at
least one of said first and second components is a metal
component.
26. A fuel cell assembly in accordance with claim 23 wherein said
brazing alloy is a first brazing alloy, said fuel assembly further
comprising an additional element wherein said additional element is
bonded to one of said first element, second element and another
element by a second brazing alloy having a liquidus temperature
less than said liquidus temperature of said first brazing
compound.
27. A fuel cell assembly in accordance with claim 23 wherein said
assembly comprises a solid oxide fuel cell.
28. A fuel cell assembly in accordance with claim 11 wherein said
liquidus temperature is about 920.degree. C.
29. A fuel cell assembly comprising a first component and a second
component wherein said first and second components are bonded by a
brazing alloy consisting essentially of silver, silicon, and a
third elemental component selected from the group consisting of
copper and vanadium, and wherein said alloy has a liquidus
temperature of less than 962.degree. C.
Description
TECHNICAL FIELD
[0002] The present invention relates to alloys for joining
materials by brazing; more particularly, to silver-containing braze
alloys for joining ceramics to metals; and most particularly, to
silver braze alloys containing silicon to lower the liquidus
temperature.
BACKGROUND OF THE INVENTION
[0003] Fuel cells which generate electric current by controllably
combining elemental hydrogen and oxygen are well known. In one form
of such a fuel cell, an anodic layer and a cathodic layer are
separated by a permeable electrolyte formed of a ceramic solid
oxide, such as yttrium-stabilized zirconium (YSZ). Such a fuel cell
is known in the art as a "solid oxide fuel cell" (SOFC). A single
cell is capable of generating a relatively small voltage and
wattage, typically between about 0.5 volt and about 1.0 volt,
depending upon load, and less than about 2 watts per cm.sup.2 of
cell surface. Therefore, in practice it is known to stack together,
in electrical series, a plurality of cells.
[0004] In a currently-preferred arrangement, each ceramic-based
fuel cell is bonded to a surrounding metal "cassette" frame to form
a fuel cell sub-assembly, using a silver/copper-based braze. As the
solid braze alloy is liquefied, the copper is rapidly oxidized to
form copper oxide which separates from the alloy, leaving
essentially pure silver as the brazing material. The copper oxide
migrates to the boundaries of the liquid and adheres to the ceramic
and the metal, providing an attachment layer for the silver.
Exemplary silver/copper and silver/vanadium braze alloys are
disclosed in International Publication No. WO 03/059843, published
24 Jul. 2003, which is incorporated herein by reference.
[0005] A problem in the use of such alloys is that the liquidus
temperature is substantially the melting point of pure silver,
962.degree. C. This temperature is high enough to preclude use of
some fuel cell materials which can be damaged by such high
temperatures. In addition, in some manufacturing schemes it is
desirable to perform two separate brazing steps, and it is further
desirable that the first brazed seal not be reliquefied when
performing the second seal. Thus, a higher temperature braze could
be used for the first seal, and a lower temperature braze for the
second seal.
[0006] What is needed in the art is a means for lowering the
liquidus temperature of a silver brazing alloy.
[0007] It is a principal object of the present invention to provide
an improved silver brazing alloy having a liquidus temperature
significantly lower than the melting temperature of silver.
SUMMARY OF THE INVENTION
[0008] Briefly described, a brazing alloy in accordance with the
invention comprises elemental silver alloyed with another element
that serves to reduce the liquidus temperature of the alloy to a
temperature below the melting point of silver. In a preferred
embodiment, a brazing alloy comprises elemental silver alloyed with
elemental silicon in a silver/silicon ratio between about 95/5 and
99/1, preferably about 97/3. Silver melts at 962.degree. C., but
small amounts of silicon alloyed with silver depress the alloy
liquidus point significantly, the liquidus temperature of a silver
and silicon eutectic alloy being about 837.degree. C. Brazing
alloys in accordance with the invention are useful in bonding
ceramics to ceramics, ceramics to metals, and metals to metals.
Such metals are preferably alumina-forming. Copper, vanadium, or
other oxygen-reactive surface bonding elements may also be
included, and use of such brazing alloys is preferably carried out
in an oxidizing atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The present invention will now be described, by way of
example, with reference to the accompanying drawings, in which:
[0010] FIG. 1 is a silver/silicon phase diagram; and
[0011] FIG. 2 is a cross-sectional view of a portion of a fuel
cell, showing bonding of a ceramic component to a metal component
by a braze alloy in accordance with the invention;
[0012] FIG. 3 is a cross-sectional view of a portion of a fuel
cell, similar to that shown in FIG. 2, but showing a second braze
alloy having a different liquidus temperature; and
[0013] FIG. 4 is a silver/ruthenium phase diagram.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Referring to FIG. 1, a phase diagram 10 for silver and
silicon alloy mixtures is conventionally displayed, having
temperature on the vertical axis as a function of weight percent
silicon on the horizontal axis. The solidus/liquidus curve 12 for
alloys of silver and silicon extends from pure silver (MP of about
962.degree. C.) to pure silicon (MP 1414.degree. C.). Addition of
relatively small amounts of silicon to pure silver serves to
depress the alloy liquidus temperature significantly; a eutectic
point 14 of about 835.degree. C. exists at a silver/silicon ratio
of about 97/3. Useful liquidus temperature depressions below the
962.degree. C. melting temperature of silver exist up to about 5%
silicon.
[0015] Referring to FIG. 4, a phase diagram 10' for silver and
ruthenium alloy mixtures of a second embodiment, including
solidus/liquidus curve 12' and eutectic point 14', is shown. In a
silver/ruthenium ratio between about 96/4 and 99/1, preferably
about 97/3, a liquidus temperature of about 920.degree. C. is
achieved. Thus, the addition of relatively small amounts of
ruthenium to pure silver similarly serves to depress the alloy
liquidus temperature below the melting point of pure silver.
[0016] Referring to FIG. 2, in a solid-oxide fuel cell assembly 16,
a ceramic-based fuel cell element 18 is bonded to a metal cassette
20 by a braze seal formed of a silver/silicon braze alloy 22 in
accordance with the invention. Preferably, the braze alloy includes
either copper or vanadium which is readily oxidized to form CuO or
V.sub.2O.sub.5 during fusion of the alloy in an oxidizing
atmosphere. The oxides separate from the alloy in known fashion and
provide an adhesion layer 24 on element 18 and cassette 20 for
attachment of the remaining silver/silicon alloy 26.
[0017] Referring to FIG. 3, it can be seen how the use of two or
more different silver/silicon braze alloys can be used so that a
additional brazed seals can be formed without re-liquefying the
already formed brazed seals. As shown, fuel cell element 18 is
bonded to a second element, such as cassette 20, by a braze seal
formed of a silver/silicon braze alloy 22 having a liquidus
temperature of less than 962.degree. C., as described above. Third
element 18' can then be bonded to element 20 (or any other element
in the fuel cell assembly) using a silver/silicon braze alloy 22'
having a different weight percentage concentration of silicon and
having a liquidus temperature lower than the melting temperature of
alloy 22.
[0018] While the invention as described above concerns a reactive
air brazing alloy, it is understood that he braze alloy can be made
from a mix of the elemental powders which would then be allowed to
alloy during the brazing process.
[0019] While the invention has been described by reference to
various specific embodiments, it should be understood that numerous
changes may be made within the spirit and scope of the inventive
concepts described. Accordingly, it is intended that the invention
not be limited to the described embodiments, but will have full
scope defined by the language of the following claims.
* * * * *