U.S. patent application number 11/289194 was filed with the patent office on 2007-05-31 for alloys for intermediate temperature applications, methods for maufacturing thereof and articles comprising the same.
Invention is credited to Aravind Dattatrayrao Chinchure, Melvin Robert Jackson, Hari N.S., Sheela Kollali Ramasesha, Kaushik Vaidya, Amitabh Verma.
Application Number | 20070122304 11/289194 |
Document ID | / |
Family ID | 38037987 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070122304 |
Kind Code |
A1 |
Ramasesha; Sheela Kollali ;
et al. |
May 31, 2007 |
Alloys for intermediate temperature applications, methods for
maufacturing thereof and articles comprising the same
Abstract
Disclosed herein is a composition comprising iron; about 18 to
about 30 wt % chromium; up to about 7 wt % tungsten; up to about
1.5 wt % manganese; up to about 1 wt % aluminum; about 0.02 to
about 0.1 wt % of a rare earth metal and/or yttrium; wherein the
weight percents are based on the total weight of the composition.
Disclosed herein too is a method comprising melting together a
composition comprising iron; about 18 to about 30 wt % chromium; up
to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to
about 1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth
metal and/or yttrium; wherein the weight percents are based on the
total weight of the composition; casting the composition; and
rolling the composition.
Inventors: |
Ramasesha; Sheela Kollali;
(Bangalore, IN) ; N.S.; Hari; (Bangalore, IN)
; Verma; Amitabh; (Sector Pi, IN) ; Chinchure;
Aravind Dattatrayrao; (Noida, IN) ; Vaidya;
Kaushik; (Bangalore, IN) ; Jackson; Melvin
Robert; (Corea, ME) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Family ID: |
38037987 |
Appl. No.: |
11/289194 |
Filed: |
November 28, 2005 |
Current U.S.
Class: |
420/40 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/06 20130101; C22C 38/22 20130101; C22C 38/18 20130101; C22C
38/005 20130101 |
Class at
Publication: |
420/040 |
International
Class: |
C22C 38/18 20060101
C22C038/18 |
Claims
1. A composition comprising: iron; about 18 to about 30 wt %
chromium; up to about 7 wt % tungsten; up to about 1.5 wt %
manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt
% of a rare earth metal and/or yttrium; wherein the weight percents
are based on the total weight of the composition.
2. The composition of claim 1, having an area specific resistivity
of about 5 to about 40 milliohm-square centimeter at 750.degree.
C., when oxidized in a sandwich configuration at 750.degree. C. for
about 1,500 hours.
3. The composition of claim 1, having an area specific resistivity
of about 20 to about 120 milliohm-square centimeter at 750.degree.
C., when oxidized in a sandwich configuration at 850.degree. C. for
about 1,500 hours.
4. The composition of claim 1, having a coefficient of thermal
expansion of greater than or equal to about 11.75 parts per million
per degree centigrade.
5. The composition of claim 1, having a coefficient of thermal
expansion of about 11.75 to about 12.6 parts per million per degree
centigrade.
6. The composition of claim 1, wherein the chromium is present in
an amount of about 25 wt %, based on the total weight of the
composition.
7. The composition of claim 1, wherein the rare earth metal is
lanthanum.
8. The composition of claim 1, comprising about 5 to about 7 wt %
tungsten.
9. The composition of claim 1, comprising about 0.5 to about 1.5 wt
% manganese.
10. The composition of claim 1, comprising about 0.5 to about 1 wt
% aluminum.
11. An article manufactured from the composition of claim 1.
12. A method comprising: melting together a composition comprising:
iron; about 18 to about 30 wt % chromium; up to about 7 wt %
tungsten; up to about 1.5 wt % manganese; up to about 1 wt %
aluminum; about 0.02 to about 0.1 wt % of a rare earth metal and/or
yttrium; wherein the weight percents are based on the total weight
of the composition. casting the composition; and rolling the
composition.
13. The method of claim 12, further comprising forging the
composition.
14. An article manufactured by the method of claim 12.
Description
BACKGROUND
[0001] This disclosure is related to ferritic stainless steels for
high temperature applications, methods for manufacturing thereof
and articles comprising the same.
[0002] Solid oxide fuel cells (SOFCs) are devices that produce
energy, usually electricity, from a variety of fuels using an
electrochemical reaction. Oxygen transfer through the electrolyte,
which improves the efficiency of energy conversion, is greatly
accelerated at temperatures above 700.degree. C. The overall fuel
to electricity conversion efficiency in SOFCs can be as high as 90%
and is not limited by classical thermodynamics for heat engines
(Carnot cycle). Due to their high exhaust gas temperature, SOFCs
have the ability to cogenerate heat and electricity. Hybrid power
generation systems integrating the SOFCs and turbines can have very
high overall system efficiencies.
[0003] SOFCs may be tubular or planar in assembly. The key
components of an SOFC are an anode, a cathode, an electrolyte,
interconnects, a manifold and seals. The cathode is largely exposed
to a hot, oxidant environment, and is generally called the air or
oxygen electrode. The temperature of the cathode feed gas is
usually about 400.degree. C. or higher. Similarly, the anode is
exposed to the fuel and is called the fuel electrode. The
interconnects interface with the anode on the fuel side and with
the cathode on the air side and are usually made using oxidation
resistant, heat resistant materials such as lanthanum chromite,
lanthanum strontium chromite, ferritic stainless steels and
chromium base alloys.
[0004] Highly oxidizing conditions prevail at the cathode at
temperatures of greater than or equal to about 850.degree. C. and
high oxygen partial pressures. These, along with humidity and
atmospheric moisture may oxidize chromium present in interconnects
to chromium oxides or hydroxide or oxyhydroxide that grow as
cathode scales and can vaporize to poison or deactivate the
cathode. Cathode scales may grow to a thickness of tens of microns
after exposure for thousands of hours in the SOFC environment in an
intermediate temperature range of about 800.degree. C. Chromium
hydroxide and oxyhydroxide are particularly volatile and may
degrade the cathode. To enhance life expectancy and operational
efficiency of the SOFC cathode it is desirable to reduce or
eliminate cathode degradation.
[0005] Current methods for minimizing cathode degradation in SOFCs
are not adequately developed and limit the useful operating life of
the SOFCs. The problem may be reduced or eliminated by frequent
maintenance or cathode scale removal. This may result in cell
stoppage and induce a significant energy penalty associated with
the power generation cycle.
[0006] Alternatively, non-chromium containing alloys and ceramic
materials with non-volatile chromium have been employed in
interconnects. However, these materials are expensive, brittle,
weak under tensile forces, or have high resistive losses making
them unsuitable for interconnect applications. Many SOFC stacks
employ interconnects and components made from alloys containing
chromium and few suitable replacement materials are available. The
problem of high cathode degradation rates has not been solved.
[0007] It is therefore desirable to use ferritic stainless steels
that can facilitate a reduction in the cathode degradation rates in
SOFC's that operate at temperatures of about 800.degree. C.
SUMMARY
[0008] Disclosed herein is a composition comprising iron; about 18
to about 30 wt % chromium; up to about 7 wt % tungsten; up to about
1.5 wt % manganese; up to about 1 wt % aluminum; about 0.02 to
about 0.1 wt % of a rare earth metal and/or yttrium; wherein the
weight percents are based on the total weight of the
composition.
[0009] Disclosed herein too is a method comprising melting together
a composition comprising iron; about 18 to about 30 wt % chromium;
up to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to
about 1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth
metal and/or yttrium; wherein the weight percents are based on the
total weight of the composition; casting the composition; and
rolling the composition.
[0010] Disclosed herein too are articles manufactured from the
composition.
DETAILED DESCRIPTION OF FIGURES
[0011] With reference now to the figures, wherein like elements are
numbered alike:
[0012] FIG. 1 is a schematic depicting one exemplary embodiment of
a solid oxide fuel cell (SOFC);
[0013] FIG. 2 is a schematic depicting the sandwich that is used
for the ASR measurements;
[0014] FIG. 3 is a depiction of the test set-up for measuring the
ASR of the ferritic stainless steels; and
[0015] FIG. 4 depicts the electrical set-up for the platinum foils
that is used for determining the ASR of the ferritic stainless
steels.
DETAILED DESCRIPTION
[0016] In the following description, like reference characters
designate like or corresponding parts throughout the several views
shown in the figures. It is also understood that terms such as
"top", "bottom", "outward", "inward", and the like are words of
convenience and are not to be construed as limiting terms. It is to
be noted that the terms "first," "second," and the like as used
herein do not denote any order, quantity, or importance, but rather
are used to distinguish one element from another. The terms "a" and
"an" do not denote a limitation of quantity, but rather denote the
presence of at least one of the referenced item. The modifier
"about" used in connection with a quantity is inclusive of the
stated value and has the meaning dictated by the context (e.g.,
includes the degree of error associated with measurement of the
particular quantity).
[0017] Disclosed herein are ferritic stainless steels that reduce
oxidation and improve chemical compatibility of the metal
interconnect in solid oxide fuel cells (SOFCs) and other high
temperature applications. The ferritic stainless steels can be
advantageously used as interconnects in a SOFC environment while
reducing degradation due to corrosion. The ferritic stainless
steels display a low oxide growth rate, can be advantageously used
for coefficient of thermal expansion (CTE) matching and have a low
total area specific resistivity (ASR) of about 5 to about 40
milliohm-square centimeter (measured at 750.degree. C.) when
subjected to oxidation at about 750.degree. C. for about 1,500
hours. The ferritic stainless steels advantageously comprise
chromium, aluminum, tungsten, manganese, rare earth elements and/or
yttrium, with the balance being iron.
[0018] With reference now to the FIG. 1, an exemplary fuel cell
system 200 comprises a fuel cell 30 having an anode 40, an
electrolyte 60, a cathode 80, an interconnect 100 and a seal 105.
The cathode 80 and the interconnect 100 are in intimate electrical
communication via contact 90. A fuel cell stack is obtained by
repeated stacking of repeating unit 180 that comprises an anode 40,
electrolyte 60, cathode 80, cathode-interconnect contact 90 and
interconnect 100. The fuel cell is encased between the end plates
120
[0019] As can be seen from the FIG. 1, the interconnect, connects
one cell to another electrically when multiple SOFCs are used in a
stack to generate electricity. Interconnects also serve as
separators for the anode and cathode gases in addition to providing
mechanical stability to the SOFC stack. Since electrical
connectivity of SOFCs is the function of interconnects, the
electrical conductivity of the materials in the interconnect has to
be high and should stay high at the operating temperature under the
cell conditions for the entire life of the SOFC. Further, the
interconnect is in physical communication with the other components
of the cell such as the cathode and anode. Seals are used to make
the fuel cell gas-tight to avoid the intermixing of fuel and
oxidant gases and the interconnects can be in physical
communication with the seals too. Thus, it is desirable for the
interconnects to be chemically inert and to have matching
coefficient of thermal expansion with the other cell components.
Even if there is reaction between the interconnect and the
electrodes, the reaction product should be a good electrical
conductor.
[0020] In one embodiment, the ferritic stainless steel used in the
interconnect comprises chromium in an amount of greater than or
equal to about 18 weight percent (wt %), based on the weight of the
ferritic stainless steel. In another embodiment, the ferritic
stainless steel comprises chromium in an amount of about 18 wt % to
about 30 wt %, based on the weight of the ferritic stainless steel.
In yet another embodiment, the ferritic stainless steel comprises
chromium in an amount of about 20 wt % to about 29 wt %, based on
the weight of the ferritic stainless steel. In yet another
embodiment, the ferritic stainless steel comprises chromium in an
amount of about 21 wt % to about 28 wt %, based on the weight of
the ferritic stainless steel. An exemplary amount of chromium is
about 20 to about 25 wt %, based on the weight of the ferritic
stainless steel. If less than 18 wt % of chromium is added, then a
continuous protective layer of chromium oxide may not be formed.
This protective layer of chromium oxide minimizes the rate of
degradation of the ferritic stainless steel. If the chromium is
added in amounts of greater than or equal to about 30 wt %, then
the ASR will increase. There is also a risk of increased
volatilization if chromium is added in amounts of greater than or
equal to about 30 wt %, based on the weight of the ferritic
stainless steel.
[0021] The aluminum can be present in amounts of up to about 1 wt
%, based on the weight of the ferritic stainless steel. In one
embodiment, the aluminum can be present in amounts of about 0.5 to
about 0.9 wt %, based on the weight of the ferritic stainless
steel. In another embodiment, the aluminum can be present in
amounts of about 0.55 to about 0.85 wt %, based on the weight of
the ferritic stainless steel. In yet another embodiment, the
aluminum can be present in amounts of about 0.5 to about 0.80 wt %,
based on the weight of the ferritic stainless steel. An exemplary
amount of aluminum is about 0.75 wt %, based on the weight of the
ferritic stainless steel. If aluminum is added in amounts of
greater than or equal to about 1.0 wt %, then too much alumina may
be formed in the ferritic stainless steel thereby increasing the
surface resistance.
[0022] Tungsten facilitates a reduction in the coefficient of
thermal expansion (CTE) of the ferritic stainless steel. The amount
of tungsten can be varied to facilitate CTE matching between the
interconnect and those components of the SOFC that it is physical
communication with. The tungsten can be present in amounts of up to
about 7 wt %, based on the weight of the ferritic stainless steel.
In one embodiment, the tungsten can be present in amounts of about
5 to about 6.8 wt %, based on the weight of the ferritic stainless
steel. In another embodiment, the tungsten can be present in
amounts of about 5.5 to about 6.5 wt %, based on the weight of the
ferritic stainless steel. An exemplary amount of tungsten is about
5 to about 7 wt %, based on the weight of the ferritic stainless
steel.
[0023] The presence of manganese in the ferritic stainless steel
facilitates the formation of a spinel phase upon oxidation. The
presence of manganese reduces the volatilization of the
chromium-containing oxides and/or hydroxides. The manganese can be
present in amounts of up to about 1.5 wt %, based on the weight of
the ferritic stainless steel. In one embodiment, the manganese can
be present in amounts of about 0.5 to about 1.35 wt %, based on the
weight of the ferritic stainless steel. In another embodiment, the
manganese can be present in amounts of about 0.6 to about 1.25 wt
%, based on the weight of the ferritic stainless steel. In yet
another embodiment, the manganese can be present in amounts of
about 0.7 to about 1.2 wt %, based on the weight of the ferritic
stainless steel. An exemplary amount of manganese is about 0.75 wt
%, based on the weight of the ferritic stainless steel.
[0024] The rare earth elements are effective in controlling
oxidation as they effectively block the grain boundary diffusion of
chromium. An exemplary rare earth element is lanthanum. Other rare
earth metals from the lanthanide and actinide series of rare earth
metals may be added to lanthanum if desired. Examples of such rare
earth metals are cerium, praseodymium, neodymium, samarium,
europium, gadolinium, uranium, neptunium, plutonium, or the like,
or a combination comprising at least one of the foregoing rare
earth metals.
[0025] It is generally desirable to add the rare earth metals in
amounts of about 0.02 wt % to about 0.1 wt %, based on the total
weight of the ferritic stainless steel. In one embodiment, the rare
earth metals can be added in amounts of about 0.05 wt % to about
0.08 wt %, based on the total weight of the ferritic stainless
steel. In another embodiment, the rare earth metals can be added in
amounts of about 0.06 wt % to about 0.075 wt %, based on the total
weight of the ferritic stainless steel. If the rare earth metals
are added in an amount of greater than or equal to about 0.1 wt %,
then the cost of processing the ferritic stainless steel
increases.
[0026] As noted above, the ferritic stainless steels can also
comprise yttrium in addition to or in lieu of the rare earth
metals. In one embodiment, yttrium can be added with the rare earth
metals to the ferritic stainless steels. In another embodiment, the
yttrium can be used to replace the rare earth metals in the
ferritic stainless steels.
[0027] In one embodiment, the rare earth metals and the yttrium can
be added in amounts of about 0.0001 wt % to about 0.1 wt %, based
on the total weight of the ferritic stainless steel. In one
embodiment, the rare earth metals and the yttrium can be added in
amounts of about 0.005 wt % to about 0.08 wt %, based on the total
weight of the ferritic stainless steel. In another embodiment, the
rare earth metals and the yttrium can be added in amounts of about
0.007 wt % to about 0.06 wt %, based on the total weight of the
ferritic stainless steel. In yet another embodiment, the rare earth
metals and the yttrium can be added in amounts of about 0.008 wt %
to about 0.05 wt %, based on the total weight of the ferritic
stainless steel.
[0028] In one embodiment, in one method of manufacturing the
ferritic stainless steel, the iron, chromium, aluminum, tungsten,
manganese, rare earth elements and/or yttrium are vacuum arc melted
followed by casting, forging and rolling into the final sheet form.
In another embodiment, the ferritic stainless steel can be
manufactured into a desired shape by other powder metallurgy based
methods including, hot pressing, hot isostatic pressing, sintering,
hot vacuum compaction, or the like. An exemplary method of
manufacturing the ferritic stainless steel is by vacuum arc melting
followed by casting forging and rolling into final sheet form.
[0029] After vacuum arc melting the material is then cast into an
ingot. The ingot may then be forged and rolled into final sheet
form. In one embodiment, the ingot can be hot rolled at a
temperature of about 1000.degree. C., followed by cold rolling to a
thickness of less than or equal to about 2.54 millimeters. During
the process of reduction in the thickness of the cross-sectional
area, periodic annealing may be performed on the ferritic stainless
steels.
[0030] The ferritic stainless steels advantageously display an area
specific resistivity (ASR) of about 5 to about 40 milliohm-square
centimeter (mohm-cm.sup.2) when used in an alloy sandwiches that
are oxidized at 750.degree. C. for 1,500 hours and an ASR of about
20 to about 120 mohm-cm.sup.2 when used in an alloy sandwiches that
are oxidized at 850.degree. C. for 1,500 hours. The aforementioned
ASR values are measured at a test temperature of 750.degree. C. As
detailed below, the alloy sandwiches contain a layer of lanthanum
strontium manganate disposed between two ferritic stainless steel
plates.
[0031] The ferritic stainless steels also advantageously display a
coefficient of thermal expansion (CTE) of about 11 to about 12.75
parts per million per degree centigrade (ppm/.degree. C.). In one
embodiment, the ferritic stainless steels display a coefficient of
thermal expansion (CTE) of about 11.75 to about 12.50 ppm/.degree.
C. In another embodiment, the ferritic stainless steels display a
coefficient of thermal expansion (CTE) of about 11.85 to about
12.25 ppm/.degree. C. The ferritic stainless steels advantageously
have a thermal expansion coefficient to match to that of the
electrolyte material that is used in commercially available SOFC's
i.e., 8% yttria stabilized zirconia (YSZ), which is about 11
ppm/.degree. C. in the temperature range of about 20 to about
800.degree. C.
[0032] The present disclosure is illustrated by the following
non-limiting example.
EXAMPLE
[0033] This example was performed to determine the area specific
resistivity (ASR), the coefficient of thermal expansion (CTE) and
the thickness of an oxidation layer formed on the ferritic
stainless steel in a solid oxide fuel cell environment. To measure
the ASR, a sandwich of an LSM (lanthanum strontium material) and
the ferritic stainless steel was created. As shown in the FIG. 2,
this Sandwich Configuration comprises a layer of LSM disposed
between two ferritic stainless steel plates. The whole assembly
shown in the FIG. 2 was oxidized at high temperatures for a certain
duration of time. The temperatures chosen were 750 and 850.degree.
C. respectively and the duration of time was 1,500 hours.
[0034] In order to sandwich the LSM between the ferritic stainless
steel plates, 10 wt % polyvinyl alcohol (PVA) is dissolved in hot
water to make a PVA solution. LSM paste was prepared with 30 wt %
of this PVA solution, that is 70 grams of LSM was mixed with 30
grams of PVA solution. The LSM paste was then applied to one
surface of a ferritic stainless steel plate and another ferritic
stainless steel plate was pressed on it. These alloy sandwiches
were then oxidized at 750.degree. C. and 850.degree. C.
respectively for 1,500 hours. These oxidation temperatures were
chosen because they are similar to the operating temperature of a
SOFC.
[0035] In order to measure the ASR, after oxidizing the sandwiches,
the top and bottom surfaces of the sandwich were polished off to
remove the oxide that is formed on the bare surfaces of the
ferritic stainless steel plates. Then the sandwich is introduced
into the measuring equipment between the platinum foils, as shown
in FIG. 3. As can be seen in the FIG. 3, the platinum foils each
having two leads are in intimate contact with the outer surfaces of
the sandwich. This is depicted clearly in the FIG. 4 where two of
the leads are connected to the top platinum foil and the other two
to the bottom platinum foil. One of the leads on top and one from
bottom are used for passing a constant current and the other pair
for measuring the voltage drop across the sandwich.
[0036] The advantages of this configuration are a) after polishing
off the oxide on the top and bottom surfaces of the sandwich, the
platinum foils make direct contact with the alloys and b) The total
ASR measured is across two ferritic stainless steel-LSM interfaces
thereby increasing the accuracy of measurement.
[0037] A Keithley programmable constant current source (model 2400)
and Keithley Nanovoltmeter (model 2182) were used for passing the
constant current and measuring voltage drop across the sample,
respectively. The voltage drop was also measured by reversing the
polarity of the constant current and the average of the two
readings was taken. This way any thermoelectric effects that may be
present because of temperature gradients in the furnace are also
annulled. The temperature was increased at a rate of 5 degrees
centigrade per minute and the data was collected at an interval of
20 degrees both during heating and cooling.
[0038] The compositions along with the ASR results for these
compositions are shown in the Table 1 below. In addition to the ASR
measurements, CTE measurements were also made using a Netzsch DIL
402C dilatometer having temperature capability from 25 to
1500.degree. C. CTE results are also shown in the Table 1
below.
[0039] In addition samples were oxidized to determine the oxide
thickness. Ferritic stainless steel pieces were coated with LSM
slurry and were oxidized at 750 and 850.degree. C. for 1,500 hours.
The oxidized alloys were mounted edge-on to determine the oxide
thickness. In order to ensure perpendicularity, a couple of
metallic clips were used. The samples supported by the clips were
inserted in the plastic cylindrical mold of 1 inch diameter. Low
viscosity epoxy resin was prepared by mixing 3 parts resin and 1
part hardener. The cylindrical molds were half filled with the
resin and kept in the vacuum desiccators. The desiccator was
evacuated using a rotary pump until the epoxy started frothing and
reached to the rim of the mold. The vacuum was broken so that resin
sank again. The process described above was repeated once again.
Finally, the mold was filled with the resin fully. The resin was
allowed to cure overnight at room temperature.
[0040] The cold mounted samples were metallographically polished.
In order to provide a leakage path for the electrical current
developed during electron microscopy, a silver contact was provided
between sample and bottom of the molded plastic. The mounted
samples along with the plastic were degassed in the oven at
105.degree. C. for 4 to 5 hours. The degassed mounted samples were
coated with gold by DC sputtering. The thickness of the gold layer
was 150 to 200 Angstroms. The oxide thickness was measured in a
scanning electron microscope (SEM) at a magnification of 3000 to
5000. Often EDS was used as an aid for thickness measurement,
wherever the boundaries of oxides were poorly defined. Thickness
was measured at a minimum of 5 locations. Oxide thickness results
are also shown in the Table 1 below. TABLE-US-00001 TABLE 1 Oxide
Oxide Thickness Thickness (.mu.m) (.mu.m) (LSM ASR @ (bare) coated)
CTE 750.degree. C. (Oxidized at (Oxidized at (775-100.degree. C.)
(mohm- 750.degree. C. for 750.degree. C. for Sample # Composition
(ppm/.degree. C.) cm.sup.2) 1,500 hours) 1,500 hours) Sample #1
Fe--25Cr--0.75Mn--0.05(La + Y)--7W 11.78 2.4 .+-. 0.4 1.6 .+-. 0.3
Sample #2 Fe--25Cr--0.75Mn--0.05(La + Y)--1Al 12.57 12 3.3 .+-. 1.1
2.1 .+-. 0.3 Sample #3 Fe--25Cr--0.75Mn--0.1(La + Y) 12.23 11 2.2
.+-. 0.5 1.9 .+-. 0.4 Sample #4 Fe--25Cr--0.75Mn--0.1(La +
Y)--7W--1Al 12.29 12 4.4 .+-. 1.1 1.9 .+-. 0.5
[0041] From the Table 1, it can be sent that the ferritic stainless
steels have CTE's that are about 11.75 to about 12.6 ppm/.degree.
C. These CTE values permit closer thermal expansion match to
electrolyte materials that are suitable for use in commercially
available SOFC's.
[0042] From the Table 1, it may also be seen that the ASR for the
disclosed compositions is about 11 to about 12 mohm-cm.sup.2. These
values of ASR render the ferritic stainless steels useful for solid
oxide fuel cells that operate at temperatures of about 800 to about
850.degree. C. The average value of the oxide thickness layer for
the LSM coated samples is about 1.9 micrometers when oxidized at
750.degree. C. for 1,500 hours.
[0043] Thus from the examples it may be seen that the ferritic
stainless steels can be advantageously used in interconnects and
other high temperature applications. They can be advantageously
used at temperatures of up to 850.degree. C. They display good
oxidation resistance leading to increased stability of the
LSM-ferritic stainless steel interface. The ferritic stainless
steel also comprises elements that permit oxidation resistance as
well as chemical compatibility with other components of a SOFC.
[0044] While the invention has been described with reference to
exemplary embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many 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.
* * * * *