U.S. patent application number 14/480339 was filed with the patent office on 2015-02-26 for multi-layer coating.
The applicant listed for this patent is Technical University of Denmark. Invention is credited to Peter Vang Hendriksen, Peter Halvor Larsen, Soeren Linderoth, Lars Mikkelsen, Mogens Mogensen.
Application Number | 20150056535 14/480339 |
Document ID | / |
Family ID | 36803466 |
Filed Date | 2015-02-26 |
United States Patent
Application |
20150056535 |
Kind Code |
A1 |
Hendriksen; Peter Vang ; et
al. |
February 26, 2015 |
MULTI-LAYER COATING
Abstract
A multi-layer coating for protection of metals and alloys
against oxidation at high temperatures is provided. The invention
utilizes a multi-layer ceramic coating on metals or alloys for
increased oxidation-resistance, comprising at least two layers,
wherein the first layer (3) and the second layer (4) both comprise
an oxide, and wherein the first layer (3) has a tracer diffusion
coefficient for cations M.sup.m+, where M is the scale forming
element of the alloy, and the second layer (4) has a tracer
diffusion coefficient for oxygen ions O.sup.2- satisfying the
following formula: .intg. ln p ( O 2 ) in ln p ( O 2 ) ex ( D O + m
2 D M ) ln p ( O 2 ) < 5 10 - 13 cm 2 / s ##EQU00001## wherein
p(O.sub.2).sub.in, p(O.sub.2).sub.ex, D.sub.M, and D.sub.O are as
defined herein. The coating may be used in high temperature
devices, particularly for coating interconnect materials in solid
oxide electrolytic devices, including solid oxide fuel cells
(SOFCs) and solid oxide electrolysis cells (SOECs).
Inventors: |
Hendriksen; Peter Vang;
(Hilleroed, DK) ; Mikkelsen; Lars; (Roskilde,
DK) ; Larsen; Peter Halvor; (Roskilde, DK) ;
Linderoth; Soeren; (Roskilde, DK) ; Mogensen;
Mogens; (Lynge, DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Technical University of Denmark |
Kgs. Lyngby |
|
DK |
|
|
Family ID: |
36803466 |
Appl. No.: |
14/480339 |
Filed: |
September 8, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12298458 |
Feb 20, 2009 |
8859116 |
|
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PCT/EP2007/003593 |
Apr 24, 2007 |
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14480339 |
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Current U.S.
Class: |
429/465 ;
204/267; 204/279; 427/126.3; 427/454 |
Current CPC
Class: |
H01M 8/0232 20130101;
H01M 8/2425 20130101; C25B 9/18 20130101; H01M 8/0217 20130101;
H01M 8/0236 20130101; H01M 2008/1293 20130101; Y10T 428/24942
20150115; H01M 4/9033 20130101; H01M 8/0228 20130101; H01M 8/0245
20130101; C25B 9/04 20130101; H01M 8/0206 20130101; H01M 8/12
20130101; H01M 8/1286 20130101; H01M 8/0215 20130101; Y02E 60/50
20130101 |
Class at
Publication: |
429/465 ;
204/279; 204/267; 427/126.3; 427/454 |
International
Class: |
H01M 8/02 20060101
H01M008/02; C25B 9/18 20060101 C25B009/18; C25B 9/04 20060101
C25B009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2006 |
EP |
06008678.2 |
Claims
1-23. (canceled)
24. A multilayer coating for a metal containing surface of an
interconnect of a solid oxide electrolytic device, the coating
comprising at least two layers: a first layer in direct contact
with the metal containing surface of the interconnect and a second
layer in contact with the surrounding atmosphere, wherein both the
first and second layers comprise an oxide; and wherein the first
layer has a tracer diffusion coefficient for cations M.sup.m+, with
M being the scale forming element of the alloy, and the second
layer has a tracer diffusion coefficient for oxygen ions O.sup.2-
satisfying the following formula: .intg. ln p ( O 2 ) in ln p ( O 2
) ex ( D O + m 2 D M ) ln p ( O 2 ) < 5 10 - 13 cm 2 / s
##EQU00005## wherein p(O.sub.2).sub.in is the oxygen partial
pressure in equilibrium between the metal containing surface and
M.sub.aO.sub.b, p(O.sub.2).sub.ex is the oxygen partial pressure in
the reaction atmosphere, D.sub.M is the tracer diffusion
coefficient of the metal cations M.sup.m+ in the first layer, and
D.sub.O is the O.sup.2- tracer diffusion coefficient in the second
layer, wherein the first layer comprises an oxide having a fluorite
structure, wherein the second layer comprises an oxide having a
spinel structure, a rock salt structure, a corundum structure, or a
wurtzite structure, and wherein the first layer, the second layer,
or both, comprises a graded composition such that the composition
varies through the layer.
25. A multilayer coating suitable for a metal containing surface of
an interconnect of a solid oxide electrolytic device, the coating
comprising at least two layers: a first layer which faces the metal
containing surface and a second layer facing the surrounding
atmosphere, wherein both the first and second layers comprise an
oxide, wherein the first layer comprises an oxide having a fluorite
structure, and wherein the second layer comprises an oxide having a
spinel structure, a corundum structure, a wurtzite structure, or a
rock salt structure, and wherein the first layer, the second layer,
or both, comprises a graded composition such that the composition
varies through the layer.
26-27. (canceled)
28. A method of forming the multilayer coating of claim 24
comprising the steps of: forming the first layer on the metal
containing surface; and depositing the second layer on the first
layer.
29. (canceled)
30. The method of claim 28, wherein the first layer is formed by
depositing the oxide on the metal containing surface by
dip-coating, slurry spraying, screen printing, spin coating, PLD,
PVD, flame spraying, EPD or spray pyrolysis, and/or wherein the
second layer is formed by PLD, PVD or by plasma spraying.
31. The method of claim 28, wherein the first layer is formed by:
depositing a metal or metal salt or metal oxide on the metal
containing surface; and reacting the metal of the metal containing
surface and the metal, metal salt or metal oxide so as to form the
first layer.
32. The method of claim 28, wherein the multilayer coating is
applied to a porous metal containing surface, wherein the first
layer is formed by: impregnation of the porous metal containing
surface with a metal, metal salt or a metal oxide; and reacting the
metal of the porous metal containing surface and metal, metal salt
or metal oxide so as to form the first layer.
33. The method of claim 28, wherein the second layer is formed by:
impregnation of the first layer with a metal, metal salt or a metal
oxide; and reacting the metal, metal salt or metal oxide so as to
form the second layer on top of the first layer.
34. The method of claim 31, wherein the deposited metal is La, Sr,
or Y.
35. The method of claim 31, wherein the deposited metal oxide is
Y.sub.2O.sub.3, SrO, La.sub.2O.sub.3, or
La.sub.1-xSr.sub.xCoO.sub.3.
36. A solid oxide fuel cell stack, comprising the multilayer
coating of claim 24.
37. A solid oxide electrolysis cell stack, comprising the
multilayer coating of claim 24.
38. A method of forming the multilayer coating of claim 25
comprising the steps of: forming the first layer on the metal
containing surface; and depositing the second layer on the first
layer.
39. The method of claim 38, wherein the first layer is formed by
depositing the oxide on the metal containing surface by
dip-coating, slurry spraying, screen printing, spin coating, PLD,
PVD, flame spraying, EPD or spray pyrolysis, and/or wherein the
second layer is formed by PLD, PVD or by plasma spraying.
40. The method of claim 38, wherein the first layer is formed by:
depositing a metal or metal salt or metal oxide on the metal
containing surface; and reacting the metal of the metal containing
surface and the metal, metal salt or metal oxide so as to form the
first layer.
41. The method of claim 38, wherein the multilayer coating is
applied to a porous metal containing surface, wherein the first
layer is formed by: impregnation of the porous metal containing
surface with a metal, metal salt or a metal oxide; and reacting the
metal of the porous metal containing surface and metal, metal salt
or metal oxide so as to form the first layer.
42. The method of claim 38, wherein the second layer is formed by:
impregnation of the first layer with a metal, metal salt or a metal
oxide; and reacting the metal, metal salt or metal oxide so as to
form the second layer on top of the first layer.
43. The method of claim 40, wherein the deposited metal is La, Sr,
or Y.
44. The method of claim 40, wherein the deposited metal oxide is
Y.sub.2O.sub.3, SrO, La.sub.2O.sub.3, or
La.sub.1-xSr.sub.xCoO.sub.3.
45. A solid oxide fuel cell stack, comprising the multilayer
coating of claim 25.
46. A solid oxide electrolysis cell stack, comprising the
multilayer coating of claim 25.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a multi-layer coating for
the protection of metals and metal alloys against oxidation at high
temperatures. The coating may be used in high temperature devices,
particularly for coating interconnect materials in solid oxide
electrolytic devices, including solid oxide fuel cells (SOFCs) and
solid oxide electrolysis cells (SOECs).
BACKGROUND ART
[0002] Applications of solid oxide electrolytic devices include the
power generation by SOFCs and production of fuel gases by SOECs. In
both devices, SOFCs and SOECs, individual cells are stacked
together, with interconnect plates separating the cells, so as to
obtain a higher energy output by electricity or by fuel gases,
respectively. The interconnect plates separate the fuel gas from
the oxidant, which is typically air, and furthermore function as
the electrical connection between individual cells in a stack.
[0003] Hence, the requirements for an interconnect plate include
long term durability, i.e. high oxidation resistance in an
oxidizing and reducing atmosphere at high temperatures, i.e. above
500.degree. C., good electrical conductivity in an oxidizing and
reducing atmosphere at high temperatures, further a thermal
expansion coefficient (TEC) matching with the cell.
[0004] Commonly, metallic materials are employed as interconnect
materials, since they possess a high thermal and electrical
conductivity, are available at low costs and easy to machine.
[0005] However, during aging under operation conditions, oxides
form on both sides of the metallic interconnect. The growth of said
oxides disadvantageously leads to an increased electrical
resistance across the interconnect plate and, thus, increased power
loss. Therefore, high temperature resistant alloys have been
suggested, which contain Si, Al and/or Cr, which form a dense
SiO.sub.2 (silica), Al.sub.2O.sub.3 (alumina) or Cr.sub.2O.sub.3
(chromia) protective oxide layer. Especially alloys forming a
chromia layer during operation have been investigated as
interconnects due to a good balance of the oxidation kinetics and
electrical conductivity of chromia, as compared to silica and
alumina. Based on all requirements of the interconnect, ferritic
iron-chromium alloys and chromium-rich alloys have so far been
considered as the most promising interconnect materials.
[0006] U.S. Pat. No. 5,608,174 discloses an oxide dispersion
strengthened chromium-rich alloy, having a chromium content of more
than 65% by weight. Said alloy forms a chromia scale during aging
under operation. The growth rate of chromia at operation
temperatures >800.degree. C. is however too high, which in turn
results in the electrical resistance across the interconnect plate
reaching unacceptable high values due to the low conductivity of
chromia.
[0007] A further problem when using chromia-forming alloys as
interconnects is the evaporation of chromium containing oxides and
oxy-hydroxides on the air side of the interconnect during
operation. Said evaporation leads to deposition of
chromium-containing oxides at the air-electrode-electrolyte
interface, which decreases the electrode performance in the long
term. This phenomenon is known as "chromium poisoning".
[0008] Attempts to avoid the high electrical resistance and
chromium poisoning from the chromia scale have been made by
designing alloys which form a duplex
Cr.sub.2O.sub.3--(Mn,Cr).sub.3O.sub.4oxide scale, with the
manganese chromium spinel positioned above a layer of chromia.
[0009] US-A1-2003/0059335 proposes a chromium oxide forming
iron-based alloy, comprising 12 to 28 wt % chromium and small
amounts of La, Mn, Ti, Si, and Al. The material is capable of
forming at its surface a MnCr.sub.2O.sub.4 spinel phase at
temperatures of 700.degree. C. to 950.degree. C.
[0010] EP-B-1298228 relates to a steel material suitable as an
interconnect material for fuel cells, the material comprising 15 to
30 wt % Cr and forming oxide films having good electrical
conductivity at 700.degree. C. to 950.degree. C.
[0011] The formed manganese chromium spinel has advantageously a
lower vaporization pressure for chromium containing species than
chromia itself, and a higher electrical conductivity. However, the
chromium containing spinel still evaporates chromium containing
species, and thus a sufficient protection cannot be realized.
Moreover, Cr-diffusion is in fact faster in the spinel than in the
chromia and thus the formation of a dublex scale leads to an
increased rate of the corrosion, thereby reducing the overall
lifetime of the device.
[0012] It has been further suggested to modify the oxide scale
grown on the alloy by applying coatings on the surface of the alloy
instead of using the alloy alone. Said coatings may reduce the
growth rate of the oxide scale, increase the electrical
conductivity of the grown oxide, and reduce the chromium
evaporation from the interconnect. The coating of the alloys may,
for example, be performed by applying a dense coating on the
interconnect, or may be done by applying a porous coating.
[0013] U.S. Pat. No. 6,054,231 discloses the application of a
metallic coating on the chromium-containing interconnect. The
coated interconnect will form a conductive oxide layer containing
chromium during aging. The metallic coating is considered to be a
sink for chromium diffusing outwards from the alloy.
[0014] The proposed coating, however, does not stop chromium
containing species from diffusing further outwards from the alloy.
Therefore, metallic coatings forming a chromium containing oxide do
not act as an effective diffusion barrier towards chromium
diffusion. Instead, the metal coating merely impedes the chromium
diffusion during the initial stages of the oxidation. Furthermore,
the metallic coating does not solve the problem regarding chromium
poisoning.
[0015] U.S. Pat. No. 5,942,349 proposes to deposit an oxide coating
on the interconnect such that a layer of chromium containing spinel
is formed in a reaction between a chromia scale formed on the
interconnect and the applied oxide coating. The coating initially
impedes chromium poisoning of the cathode by catching chromium from
the interconnect in the coating forming the spinel.
[0016] However, the proposed coating does also not act as a
sufficient diffusion barrier for chromium from the interconnect.
The oxide layer formed on the interconnect will continue to grow in
thickness and thereby result in an increasing electrical resistance
across the interconnect plate. Furthermore, Cr-poisoning will occur
during long term operation, since the formed spinel becomes itself
chromium rich and the respective oxides evaporate therefrom into
the air-electrode-electrolyte interface.
[0017] Coatings of a similar kind, where a spinel is formed in a
reaction between the interconnect and an oxide coating have been
proposed in DE-A1-10306649. Said spinel is initially chromium free
due to a reaction between the alloy and a spinel forming element in
the coating.
[0018] However, this coating nevertheless suffers from the above
described problems, since the chromium transport from the alloy is
not entirely stopped and the reaction layer, although being
initially free from chromium, will eventually contain chromium.
Thus, Cr-poisoning and increasing electrical resistance will be the
result during long term operation. Said coating is, thus, not
suitable for applications requiring a very long durability of SOFC
and SOEC stacks.
[0019] Furthermore, porous coatings of conductive oxides with a
perovskite structure have been applied on interconnects as coatings
to increase the electrical conductivity of the formed oxide scale
and to stop the chromium poisoning, as described in e.g. Y. Larring
et al., Journal of the Electrochemical Society 147 (9); 3251-3256
(2000). These coatings have the same drawbacks as mentioned in the
above examples.
[0020] US-A1-2003/0194592 discloses an interconnect structure for
solid oxide electrolytic devices with a coating consisting of two
layers. The first layer comprises a Cr-containing electronic
conductive oxide covered by a second layer, which acts as a
diffusion barrier for oxygen. The second layer also stops chromium
diffusion from the first layer. The second layer is a metallic
layer, preferably a platinum layer. However, platinum is
undesirably expensive, making a commercialization of SOFC and SOEC
technology cumbersome.
[0021] WO-A1-2006/059942 relates to a strip for use as an
electrical contact consisting of a metallic base material which is
coated with a metallic layer based on a metal or metal alloy, and
further with at least one reactive layer containing at least one
element or compound which forms a spinel and/or perovskite
structure with the metal or metal alloy when oxidized.
[0022] The metal layer coating allows a tailor made
perovskite/spinel layer due to a precise control the amount of
different elements contained in the metal layer so as to be
independent from the composition of the metallic base material.
When oxidized, a single perovskite/spinel layer is formed on the
metallic base material, which provides a surface with high
electrical conductivity and a low contact resistance. Said layer is
however insufficient to prevent the further growth of the oxide
layer during operation. Furthermore, if a chromium-containing
metallic material is employed either as the metallic base material
or as a component of the metallic layer, chromium-poisoning will
still occur.
[0023] WO-A1-2006/059943 discloses a fuel component consisting of a
metallic base material coated with at least one metallic layer
based on a metal or metal alloy, and at least one reactive layer
comprising at least one element or compound which forms at least
one complex mixed oxide with the metal or metal alloy when
oxidized.
[0024] The precise composition of the coating can be tailor-made to
achieve the exact formation of the wanted complex metal oxide
structure which may be in form of a spinel, perovskite and/or any
other ternary or quaternary metal oxide structure upon oxidation
with the desired properties, such as good conductivity and good
corrosion resistance.
[0025] However, the formed oxide layer is insufficient to prevent
the further growth of the oxide layer during operation of the fuel
component. If furthermore a chromium-containing metallic material
is employed as the metallic base material or metallic coating
layer, chromium-poisoning will still occur.
[0026] The long term durability of the interconnects described in
the prior art up to date is not sufficient for many applications.
The use of specifically designed alloys for interconnect materials
does not eliminate the problem of oxide growth on the interconnect,
considerably resulting in an insufficient life time when the
interconnects are used in solid oxide cells or the like. Moreover,
if chromium-containing metallic materials are employed, which are
so far the most preferred materials for interconnects, chromium
poisoning of the electrode will still occur; the use of the so far
proposed coatings on said alloys cannot eliminate the undesired
oxide growth, and does not prevent chromium poisoning. Further, the
use of expensive metals, such as platinum, although leading to
better results, is not feasible for the commercial potential of
solid state devices, such as SOFCs and SOECs, due to the high
price.
[0027] Alloys utilized for high temperature applications often form
a protective silica layer, alumina layer or chromia layer to
protect the alloy against further oxidation. Coatings to be applied
on alloys to increase the oxidation protection have been suggested
in prior art. These include coatings in the ternary phase system
Ni--Pt--Al, MCrAlY coatings, TBC coatings, diffusion coatings etc.
as described in e.g. J. R. Nicholls, JOM-Journal of the Minerals
Metals & Materials Society 52 (1); 28-35 (2000).
OBJECT OF THE PRESENT INVENTION
[0028] It is the object of the present invention to overcome the
problems of the prior art coatings and to provide a multi-layer
coating suitable for metal containing surfaces for high temperature
applications, the coating ensuring a long term durability of, for
example, metallic interconnects in SOECs and SOFCs, to provide a
SOEC and a SOFC comprising said coating, and further to provide a
method for producing said coating.
BRIEF DESCRIPTION OF THE INVENTION
[0029] The above object is achieved by a multilayer coating
suitable for metal containing surfaces comprising at least two
layers, wherein the first layer (3) which faces the metal
containing surface and the second layer facing the surrounding
atmosphere (4) both comprise an oxide, and wherein the first layer
(3) has a tracer diffusion coefficient for cations M.sup.m+, (M is
the scale forming element of the alloy), and the second layer (4)
has a tracer diffusion coefficient for oxygen ions O.sup.2-
satisfying the following formula:
.intg. ln p ( O 2 ) in ln p ( O 2 ) ex ( D O + m 2 D M ) ln p ( O 2
) < 5 10 - 13 cm 2 / s ##EQU00002##
wherein p(O.sub.2).sub.in is the oxygen partial pressure in
equilibrium between the metallic substrate and M.sub.aO.sub.b,
p(O.sub.2).sub.ex is the oxygen partial pressure in the reaction
atmosphere, D.sub.M is the tracer diffusion coefficient of the
metal cations M.sup.m+ in the first layer (3), and D.sub.O is
O.sup.2- tracer diffusion coefficient in the second layer (4).
[0030] Said object is further achieved by a method of forming the
above multilayer coating comprising the steps of: [0031] forming
the first layer (3) on the metal surface; and [0032] depositing the
second layer (4) on the first layer (3).
[0033] Said object is finally achieved by a solid oxide fuel cell
stack and a solid oxide electrolysis cell stack comprising the
above multilayer coating.
[0034] Preferred embodiments are set forth in the subclaims.
FIGURES
[0035] The invention will in the following be explained with
reference to FIG. 1 which illustrates a multi-layer coating in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0036] In the following, the invention will be described in more
detail.
[0037] The multilayer coating suitable for metal containing
surfaces in accordance with the present invention comprises at
least two layers, wherein the first layer (3) which faces the metal
containing surface and the second layer facing the atmosphere (4)
both comprise an oxide, and wherein the first layer (3) has a
tracer diffusion coefficient for cations M.sup.m+, (where M is the
scale forming element of the alloy), and the second layer (4) has a
tracer diffusion coefficient for oxygen ions O.sup.2- satisfying
the following formula:
.intg. ln p ( O 2 ) in ln p ( O 2 ) ex ( D O + m 2 D M ) ln p ( O 2
) < 5 10 - 13 cm 2 / s ##EQU00003##
wherein p(O.sub.2).sub.in is the oxygen partial pressure in
equilibrium between the metallic substrate and M.sub.aO.sub.b,
p(O.sub.2).sub.ex is the oxygen partial pressure in the reaction
atmosphere, D.sub.M is the tracer diffusion coefficient of the
metal cations M.sup.m+ in the first layer (3), and D.sub.O is
O.sup.2- tracer diffusion coefficient in the second layer (4).
[0038] The first layer (3) is capable of minimizing the outward
diffusion of cations, while the second layer (4) minimizes the
inward diffusion of oxygen ions. Due to said structure, oxide scale
growth on the metal containing surfaces can be effectively
suppressed.
[0039] In the above formula, the tracer diffusion coefficient for
cations and the tracer diffusion coefficient for oxygen ions
satisfy said formula in a temperature range of from about 500 to
about 1000.degree. C. While the coefficients may satisfy said below
and/or above said temperature range, depending on the respective
values, it is however not critical for the present invention.
[0040] The oxygen tracer diffusion coefficient as referred to
throughout the present invention can be measured in independent
tests, as described in detail in R. A. De Souza et al., Solid State
Ionics, 106 (3-4): 175 (1998). The coefficient is determined by
means of the Isotopic Exchange Depth Profile method (IEDP).
.sup.18O/.sup.16O exchange anneals are performed at different
temperatures at P.sub.O2 of about 1 atm, and the subsequent
.sup.18O diffusion profiles are determined by Secondary Ion Mass
Spectroscopy (SIMS).
[0041] The cation tracer diffusion coefficient as referred to
throughout the present invention can be measured by SIMS, as
described in O. Schulz et al., Physical Chemistry Chemical Physics,
5 (11): 2008 (2003).
[0042] How to measure said tracer diffusion coefficients is
furthermore well known to a person skilled in the art.
[0043] Referring to FIG. 1, a system in accordance with the present
invention is illustrated, having a multi-layer coating (2) on top
of the metallic substrate, (1) for example a metallic interconnect.
The multi-layer coating comprises a layer (3) closest to the
interconnect and a layer (4) closest to the atmosphere of exposure
(5). The layer closest to the interconnect has the property to
inhibit cation diffusion, while the layer closest to the atmosphere
has the property to inhibit transport of oxygen (molecules and
ions). The coated interconnect may be preferably used in
applications working in the temperature range of 500-1000.degree.
C.
[0044] The invention is based on the principle that oxygen ions
diffuse from the atmosphere, while cations M.sup.m+ diffuse from
the metallic surface such that an oxide M.sub.aO.sub.b will be
formed. Due to the respective diffusion coefficients of the first
and second layer satisfying the above formula, the growth of the
oxide can be effectively reduced.
[0045] The first layer (3) which faces the metal surface comprises
an oxide and preferably has a low cation tracer diffusion
coefficient, i.e. less than 10.sup.-15 cm.sup.2/s, and more
preferably less than 10.sup.-17 cm.sup.2/s.
[0046] The second layer (4) comprises an oxide and preferably has
an oxygen ion tracer diffusion coefficient of less than 10.sup.-15
cm.sup.2/s, more preferably of less than 10.sup.-17 cm.sup.2/s.
[0047] In a preferred embodiment, the cation tracer diffusion
coefficient is the chromium tracer diffusion coefficient of the
respective layer.
[0048] According to a preferred embodiment, oxides with a
perovskite structure or fluorite structure may be used as layer (3)
closest to the metallic substrate, since oxides with a perovskite
structure or fluorite structure are generally poor cation
conductors.
[0049] According to the invention, the perovskite can be any
perovskite of the formula ABO.sub.3, where A, and B are cations. A
is a member of the "scandium family" (Y, La and the lanthanides (Ce
to Yb)) or an alkaline earth element (Mg, Sr, Ca, Ba), or mixtures
thereof, and B is a transition metal belonging to either the first
or second series, or Ce, Al, Ga, Sn, In, or a mixture thereof.
Preferably B is selected from the group consisting of Ti, V, Cr,
Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo and Ce. Of particular interest
are the perovskites with B.dbd.Ti, V, Cr and Fe.
[0050] Preferred perovskites are selected from the group consisting
of LaCrO.sub.3, YCrO.sub.3, SrTiO.sub.3, LaTiO.sub.3, YTiO.sub.3,
LaFeO.sub.3, YFeO.sub.3, LaVO.sub.3 and YVO.sub.3, optionally doped
with Sr, Ca, Ba and/or Mg. It is also preferred that the perovskite
is composed of SrVO.sub.3, (La,Sr)VO.sub.3 or (La,Sr)(Cr,V)O.sub.3.
In a preferred embodiment the perovskite is composed of SrTiO.sub.3
which is optionally doped with Nb or La.
[0051] According to the invention, the fluorite can be any fluorite
of the formula AO.sub.2 where A is a cation. Preferred fluorites
are selected from the group consisting of stabilized zirconia
(yttria, calcia, or magnesia stabilized zirconia).
[0052] In a further preferred embodiment, layer (4), being closest
to the exposing atmosphere, comprises an oxide having a spinel
structure, a rock salt structure, a corundum structure, or a
wurtzite structure since oxides having a said structures are
generally poor oxygen ion conductors.
[0053] According to the invention, the spinel can be any spinel of
the formula AB.sub.2O.sub.4, where A and B are transition metals
belonging to either the first or second series, or Ce, Al, Ga, Sn
or In, or a mixture hereof. Preferably, A and B are selected from
the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb,
Mo, Ce, and mixtures thereof. Of particular interest are the
spinels (Mn, Ni, Co, Cr, Fe, Cu).sub.3O.sub.4, with
(Mn,Cr,Co).sub.3O.sub.4 being most preferred.
[0054] According to the invention, the rock salt can be any rock
salt of the formula AB, where A is a cation and B is oxygen.
Particularly preferred are rock salts selected from the group
consisting of CaO, SrO, BaO, FeO, CoO, CdO, MgO, and NiO.
[0055] According to the invention, the corundum can be any corundum
of the formula A.sub.2B.sub.3, where A is a cation and B is oxygen.
Particularly preferred are corundums selected from the group
consisting of .alpha.-Al.sub.2O.sub.3, Ti.sub.2O.sub.3,
V.sub.2O.sub.3, .alpha.-Mn.sub.2O.sub.3, .alpha.-Ga.sub.2O.sub.3,
and .alpha.-Fe.sub.2O.sub.3.
[0056] According to the invention, the wurtzite can be any wurtzite
of the formula AB, where A is a cation and B is oxygen.
Particularly preferred are wurtzites selected from the group
consisting of BeO, ZnO.
[0057] In a more preferred embodiment of the invention, layer (4)
of the coating comprises a spinel in combination with layer (3)
comprising a perovskite. Said combination is especially suitable as
a multi-layer coating for interconnects in SOFCs and SOECs due to a
reasonable high electronic conductivity of oxides with the spinel
and the perovskite structure, for a large range of different
element combinations. The spinel structure inhibits oxygen
diffusion whereas the perovskite structure inhibits cation
diffusion from the interconnect though the layer. Also preferred
for metallic interconnects in SOFCs and SOECs is layer (4) of the
coating comprising a rock salt in combination with layer (3)
comprising a perovskite. The rock salt structure also efficiently
inhibits oxygen diffusion, as described for the spinel structure
above.
[0058] For high temperature applications, it is further preferred
that layer (3) of the coating comprises a perovskite in combination
with layer (4) comprising a corundum structure, or layer (4)
comprising a wurtzite structure.
[0059] Alternatively, preferred for high temperature applications
is layer (3) of the coating comprises a fluorite in combination
with layer (4) comprising a rock salt structure, a corundum
structure, a wurtzite structure, or a spinel structure.
[0060] Therewith, oxide growth is efficiently inhibited on the
metal containing surface, contributing to a longer life time of the
high temperature application comprising the coated metal containing
layer.
[0061] The exact combination of specific materials depends on the
respective tracer diffusion coefficients. If the diffusion of
cations through layer (3) is very low, the requirements for the
transport properties of layer (4) become relatively easy to fulfill
(c.f. the equation above). This allows for more freedom regarding
the materials for each layer for a given metal containing surface
in the light of the desired application.
[0062] The above compositions of the spinel and perovskites are
listed as stoichiometric oxides. The stoichiometry of the materials
utilized in the multi-layer coat can, however, be
non-stoichiometric without departing from the scope of the present
invention. For example, the perovskite may be sub-stoichiometric,
i.e. a perovskite AB.sub.yO.sub.3, where y<1, e.g.
LaCr.sub.0.99O.sub.3. In this case, possible oxide scales formed on
the interconnect during aging react with the LaCr.sub.0.99O.sub.3
so that the cations from the oxide scale are incorporated in the
perovskite structure on the B-site. As a result the thickness of
the thermally grown oxide scale can be reduced. The materials may
also be doped with various elements.
[0063] The individual oxide layers of the coating may have a graded
composition such that the composition varies through the oxide
layer. Furthermore, the two oxide layers may be graded such that
the composition of the one layer gradually changes into the
composition of the other layer. In another preferred embodiment,
either oxide layer contains secondary phases, which exist in the
grain boundaries of the oxide. The two individual layers can also
be a mixture of oxides with low ionic (cation or oxide) transport
and oxides exhibiting high electronic conduction.
[0064] It is preferred that the coating has two layers. However,
additional layers may also be part of the coating besides layers
(3) and (4). These layers may be positioned between the
interconnect and layer (3), between layer (3) and layer (4), or
between layer (4) and the atmosphere. These layers may provide
additional properties to the coating, i.e. function as additional
diffusion barrier layers, adherence layers, doping layers, strain
compensating layers or the like.
[0065] In case the multi-layer coating is applied on metallic
interconnects for SOFCs and SOECs, both layers in the multi-layer
coating are electronically conductive. In a preferred embodiment,
the area specific resistance of the coating is less than 0.05
.OMEGA.cm.sup.2 at 600.degree. C.
[0066] The multi-layer coating may of course be applied on both
sides of the interconnect, i.e. on the air and the fuel side of the
interconnect, if desired.
[0067] The thickness of the multi-layer coating is preferably less
than 50 .mu.m, and more preferably less than 20 .mu.m.
[0068] The thickness of the first layer (3) is preferably less than
25 .mu.m, and more preferably less than 10 .mu.m.
[0069] The thickness of the second layer (4) is preferably less
than 25 .mu.m, and more preferably less than 10 .mu.m.
[0070] The present invention further provides a method of forming a
coating suitable for metal containing surfaces comprising at least
two layers, wherein the first layer (3) which faces the metal
containing surface and the second layer facing the atmosphere of
exposure (4) both comprise an oxide, and wherein the first layer
(3) has a tracer diffusion coefficient for cations M.sup.m+, (M is
the scale forming element of the alloy), and the second layer (4)
has a tracer diffusion corefficient for oxygen ions O.sup.2-
satisfying the following formula:
.intg. ln p ( O 2 ) in ln p ( O 2 ) ex ( D O + m 2 D M ) ln p ( O 2
) < 5 10 - 13 cm 2 / s ##EQU00004##
wherein p(O.sub.2).sub.in is the oxygen partial pressure in
equilibrium between the metallic substrate and M.sub.aCo.sub.b,
p(O.sub.2).sub.ex is the oxygen partial pressure in the reaction
atmosphere, D.sub.M is the tracer diffusion coefficient of the
metal cations M.sup.m+ in the first layer (3), and D.sub.O is
O.sup.2- tracer diffusion coefficient in the second layer (4); the
method comprising the steps of: [0071] forming the first layer (3)
on the metallic substrate; and [0072] depositing the second layer
(4) on the first layer (3).
[0073] According to a preferred embodiment, oxides with a
perovskite structure may be used as layer (3). The perovskite layer
may be formed in a reaction between the interconnect and a
deposited metal, such as La, Sr, Y, or in a reaction between the
interconnect and a deposited metal salt or metal-oxide, such as
Y.sub.2O.sub.3, SrO, La.sub.2O.sub.3, La.sub.1-xSr.sub.xCoO.sub.3.
Alternatively, any other structure for layer (3) as described above
for the dual layer of the present invention may be used.
[0074] The oxide may also be deposited on the interconnect by any
other method known in the art, including dip coating, slurry
spraying, screen printing, spin coating, electroplating, flame
spraying, EPD, electrolytic deposition, physical or chemical
deposition from an oxide target, sputtering, electrostatic
spraying, plasma spraying, laser techniques, or spray
pyrolysis.
[0075] In another preferred embodiment, layer (4), being closest to
the atmosphere of exposure, comprises an oxide having a spinel
structure, since oxides having a spinel structure are generally
poor oxygen ion conductors. The spinel layer may be formed on top
of the perovskite layer in a reaction between the perovskite and
precursor materials. Said precursors include metals, metal-salts
and oxides. The spinel and the perovskite may also be formed in a
reaction during a heat treatment. The spinel layer may be deposited
on the perovskite after a heat treatment of the
interconnect-perovskite. Alternatively, the spinel layer may be
deposited on the perovskite layer without any prior heat treatment
of the perovskite layer. The spinel layer may furthermore be
deposited on the perovskite layer by similar techniques as
described above for the perovskite layer.
[0076] Of course, the same applies if oxides other than a
perovskite and/or a spinel are employed for layers (3) and (4).
[0077] The coating may be formed in air, or alternatively in
atmospheres containing less oxygen. The conditions during the
formation of the multi-layer coating may also include a sequential
treatment in different atmospheres and at different temperatures,
depending on the materials used.
[0078] The coating may be sintered in air, or alternatively in
atmospheres containing less oxygen. The sintering conditions may
also include a sequential treatment in different atmospheres and at
different temperatures, depending on the materials used.
[0079] The surface of the metal containing surface may be treated
in various ways prior to deposition of the coating. The treatments
include grinding, polishing, pickling, sand blasting, etc.
Furthermore, the metal containing surface may be pre-oxidized to
form a small amount of oxide prior to coating. The pre-treatment of
the metal containing surface may also include pre-oxidation after
deposition of oxides, e.g. reactive elements to improve adhesion,
or dopants to improve the electrical conductivity and the like.
[0080] The metal containing surface may be the surface of any metal
or metal alloy. Preferably, the coating of the present invention is
applied to surfaces of metal containing interconnects. In another
preferred embodiment the metal containing material is a porous
metal or metal alloy support.
[0081] According to a preferred embodiment, the multi-layer coating
is applied on metallic substrates as an oxidation-barrier for
metals or alloys such that the coated metal or alloy substrate
possesses a high oxidation-resistance. Multi-layer coatings used
for interconnects may be utilized for this embodiment as well. In
addition, other materials may be utilized in the multi-layer
coating for this embodiment, since the multi-layer coating is not
necessarily electronically conductive in this case. This opens the
possibility of using other ceramic materials.
[0082] According to the invention, the diffusion preventing effect
is achieved by the combination of at least two separate layers.
Thereby, each layer can be optimized with regard to its desired
characteristics, cation diffusion prevention and oxygen diffusion
prevention, which reduces the minimum requirements for each layer,
as compared to a single layer oxide which has to fulfill both
properties at the same time. It is thus possible to use a great
variety of known electron conducting materials for each layer,
which may be tailored depending on the intended purpose.
[0083] The coating of the present invention comprises the oxidation
resistant properties so far required for metallic substrates during
operation, so that the coated metallic substrates do not
necessarily need to possess said oxidation resistance properties,
i.e. the metallic substrate does not need to grow an oxidation
resistant chromia, silica or alumina scale during operation.
Instead, the metallic substrate possesses as the minimum
requirement only the necessary mechanical properties for the
application in question. Therefore, a large freedom with regard to
the selection of metals or alloys is given.
[0084] Further advantages of the multi-layer coating of the present
invention, when applied in SOFCs and SOECs, include the prevention
of the problems encountered in the prior art, such as Cr-poisoning,
or a large increase of the interface resistance. According to the
invention, Cr-poisoning is effectively inhibited and the rate of
increase of electrical resistance is strongly reduced.
[0085] The coating of the present invention may therefore be
advantageously used as coatings for interconnects in SOFCs and
SOECs, where the coating can decrease the electrical degradation
observed for interconnects and at the same time inhibit chromium
poisoning. If the coating is used in SOFC and SOEC applications,
the coating must be electronically conductive. However, the coating
is not limited to these applications, but may be employed in high
temperature oxidation applications in general.
[0086] The SOFCs and SOECs, comprising the coating of the present
invention, possess an increased lifetime due to less oxidation of
the interconnects and less Cr-poisoning of the electrodes.
Furthermore, since more flexibility in the choice of the materials
for the interconnect and design thereof can be realized, the SOFCs
and SOECs are more cost effective.
[0087] Furthermore, prior to the deposition or after the deposition
of the multi-layer coating, the metallic substrate (interconnect or
general metallic substrate) may be shaped, e.g. by pressing
methods, or a part of the substrate may be removed, e.g. by etching
methods, depending on the desired application.
[0088] In the following, the invention will be illustrated by
examples. Alternative embodiments and examples exist without
departing from the scope of the present invention.
EXAMPLES
Example 1
[0089] A dual layer coating was deposited on a
Cr.sub.2O.sub.3-forming Fe-22Cr alloy. The first oxide layer was
deposited by PLD on the alloy surface with the composition
La.sub.0.95Sr.sub.0.05CrO.sub.3 with a thickness of 5 .mu.m, said
composition having a perovskite structure. Afterwards, a 5 .mu.m
MnCr.sub.2O.sub.4 layer having a spinel structure was deposited on
the perovskite layer by PLD so as to form a dual layer coating. The
Cr tracer diffusion coefficient for La.sub.0.95Sr.sub.0.05CrO.sub.3
has been measured to be 1.07.times.10.sup.-17 cm.sup.2/s at
1000.degree. C. (N. Sakai et al., Solid State Ionics, 135 (2000) p.
469). The oxygen tracer diffusion coefficient of the
MnCr.sub.2O.sub.4 layer has been measured to be 6.times.10.sup.-15
cm.sup.2/s at 800.degree. C. (N. Sakai et al., Solid State Ionics,
176 (2005) p. 681).
Example 2
[0090] A dual-layer coating was formed on a ferritic Fe--Cr
interconnect. The first oxide layer was directly deposited on the
metal by slurry spraying LaCrO.sub.3 having a perovskite structure.
Afterwards, a 5 .mu.m thin MnCr.sub.2O.sub.4 layer having a spinel
structure was deposited on the perovskite layer by PLD so as to
form the dual-layer coating.
Example 3
[0091] A coating as described in Example 2 was formed, followed by
deposition of a layer of MnCo.sub.2O.sub.4 by PLD on top of the
spinel layer.
Example 4
[0092] A dual-layer coating was formed on a Fe--Cr-based
interconnect. The first oxide layer was formed by depositing a
metallic La layer by PLD, followed by a reaction between the La
layer and the interconnect at 1000.degree. C. in air, thereby
forming LaCrO.sub.3 having a perovskite structure. Afterwards, a
thin MnCr.sub.2O.sub.4 layer having a spinel structure was
deposited on the perovskite layer by PLD.
Example 5
[0093] A dual-layer coating was formed on a ferritic alloy. The
first oxide layer was formed as described in Example 1, followed by
depositing Mn.sub.2O.sub.3 on top of the perovskite layer by slurry
spraying. Afterwards, a spinel layer was formed by the reaction
between the perovskite layer and the deposited oxide at 950.degree.
C. in air.
Example 6
[0094] Same as Example 5, wherein Co.sub.3O.sub.4 was used instead
of Mn.sub.2O.sub.3.
Example 7
[0095] Same as Example 5, wherein Fe.sub.2O.sub.3 was used instead
of Mn.sub.2O.sub.3.
Example 8
[0096] Same as Example 2, wherein the surface finish of the
metallic alloy before deposition of the coating was pre-oxidized at
about 900.degree. C. in an H.sub.2/H.sub.2O atmosphere for 30
min.
Example 9
[0097] A small amount of Ni(NO.sub.3).sub.2 was applied onto the
interconnect of Example 2 by dip coating the metallic interconnect
in a nitrate-solution prior to the formation of the dual-layer
coating. The interconnect with the applied Ni(NO.sub.3).sub.2 was
pre-oxidized at about 900.degree. C. in air for 24 h.
Example 10
[0098] A coating was formed on a Fe--Cr metallic substrate. A layer
of stabilized zirconia (e.g. yttria stabilized zirconia) having a
fluorite structure was deposited on the metallic substrate by PLD.
Afterwards, a thin MnCr.sub.2O.sub.4 layer having a spinel
structure was deposited on the YSZ layer by PLD so as to form a
dual-layer coating. The tracer diffusion coefficient of Ti has been
measured to be 5.times.10.sup.-16 cm.sup.2/s at 1200.degree. C. (K.
Kowalski et al., Journal of the European Ceramic Society, 20 (2000)
p. 951). The tracer diffusion coefficients of other transition
metals (e.g. Cr, Fe, and Al) will be of similar magnitude.
Example 11
[0099] A dual layer coat was formed on the surface in a Fe22Cr
porous metal support for SOFC by a two step impregnation. First a
nitrate solution of LaCrO3 is impregnated into the structure by
vacuum impregnation. After a subsequent heat treatment to
800.degree. C. a nitrate solution of MnCr2O4 is impregnated. The
protective dual coat layer is completed by a heat treatment to
800.degree. C.
Example 12
[0100] As Example 11 but using MnCo.sub.2O.sub.4 for the second
layer.
[0101] The coating system of the present application can also be
applied as an oxidation resistant coating of metallic substrates
for other high temperature (>500.degree. C.) applications than
the applications as SOFC and SOEC interconnects outlined above.
Alloys utilized for high temperature applications often form a
protective silica layer, alumina layer or chromia layer to protect
the alloy against further oxidation. By using an oxidation
resistant coating, the metallic substrate does not need to be
oxidation resistant itself. This means that a larger number of
metals and alloys may be used for high temperature applications.
Importantly, the coating does not in general need to be
electronically conductive for this purpose.
* * * * *