U.S. patent application number 13/850410 was filed with the patent office on 2013-08-29 for plasma spray method for the manufacturing of an ion conducting membrane and an ion conducting membrane.
This patent application is currently assigned to Sulzer Metco AG. The applicant listed for this patent is Forschungszentrum Julich GmbH, Sulzer Metco AG. Invention is credited to Malko Gindrat, Andreas Hospach, Georg Mauer, Karl-Heinz Rauwald, Detlev Stover, Robert Vassen, Konstantin von Niessen.
Application Number | 20130220126 13/850410 |
Document ID | / |
Family ID | 47603499 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130220126 |
Kind Code |
A1 |
Hospach; Andreas ; et
al. |
August 29, 2013 |
PLASMA SPRAY METHOD FOR THE MANUFACTURING OF AN ION CONDUCTING
MEMBRANE AND AN ION CONDUCTING MEMBRANE
Abstract
A plasma spray method for the manufacture of an ion conducting
membrane, in particular of a hydrogen ion conducting membrane or of
an oxygen ion conducting membrane is suggested. In which method the
membrane is deposited as a layer (11) on a substrate (10) in a
process chamber, wherein a starting material (P) is sprayed onto a
surface of the substrate (10) by means of a process gas (G) in the
form of a process beam (2). The starting material is injected into
a plasma at a low process pressure which is at most 10000 Pa and is
partially or completely melted there. In accordance with the
invention the substrate (10) has pores (30) which are connected
amongst one another so that the substrate (10) is gas permeable and
a portion of an overall pore area of an overall area of the coating
surface (31, 131) amounts to at least 30%, in particular to at
least 40%.
Inventors: |
Hospach; Andreas; (Julich,
DE) ; Vassen; Robert; (Herzogenrath, DE) ;
Mauer; Georg; (Tonisvorst, DE) ; Rauwald;
Karl-Heinz; (Julich, DE) ; Stover; Detlev;
(Niederzier, DE) ; von Niessen; Konstantin;
(Buttwil, CH) ; Gindrat; Malko; (Wohlen,
CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Forschungszentrum Julich GmbH;
Sulzer Metco AG; |
|
|
US
US |
|
|
Assignee: |
Sulzer Metco AG
Wohlen
CH
Forschungszentrum Julich GmbH
Julich
DE
|
Family ID: |
47603499 |
Appl. No.: |
13/850410 |
Filed: |
March 26, 2013 |
Current U.S.
Class: |
96/4 ; 427/446;
427/453 |
Current CPC
Class: |
C23C 14/027 20130101;
B01D 53/22 20130101; C23C 14/083 20130101; Y02T 50/60 20130101;
C23C 28/30 20130101; C23C 4/134 20160101; Y02T 50/6765 20180501;
C23C 14/228 20130101; Y10T 428/24174 20150115 |
Class at
Publication: |
96/4 ; 427/446;
427/453 |
International
Class: |
C23C 4/12 20060101
C23C004/12; B01D 53/22 20060101 B01D053/22 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2012 |
EP |
12156756.4 |
Claims
1. A plasma spray method for the manufacture of an ion conducting
membrane, in particular of a hydrogen ion conducting membrane or of
an oxygen ion conducting membrane, wherein the membrane is
deposited as a layer (11, 111) on a substrate (10,110) in a process
chamber (12), wherein a starting material (P) is sprayed onto a
coating surface (31, 131) of the substrate (10, 110) by means of a
process gas (G) in the form of a process beam (2), wherein the
starting material is injected into a plasma at a low process
pressure which is at most 10000 Pa and is partially or completely
melted there, characterized in that the substrate (10, 110) has
pores (30) which are connected amongst one another so that the
substrate (10, 110) is gas permeable and a portion of an overall
pore area of an overall area of the coating surface (31, 131)
amounts to at least 30%, in particular to at least 40%.
2. A plasma spray method in accordance with claim 1, characterized
in that the pores (30) have a mean pore size of at least 1
micrometer.
3. A plasma spray method in accordance with claim 1, characterized
in that the substrate (10, 110) has a useful porosity of at least
20%, in particular of at least 30% with respect to an overall
volume of the substrate (10, 110).
4. A plasma spray method in accordance with claim 1, characterized
in that micro-passages (140) are introduced into the substrate
(110) before or after the coating for the improvement of a gas flow
possibility in the direction of the coating surface (131).
5. A plasma spray method in accordance with claim 4, characterized
in that the micropassages (140) are orientated in the direction of
the coating surface (131) from a rear substrate side (132) lying
opposite the coating surface (131) and end before the coating
surface (131).
6. A plasma spray method in accordance with claim 1, characterized
in that the substrate (10, 110) is manufactured from an iron alloy
which has a portion of chrome which is larger than 20 weight
percent and in particular is larger than 25 weight percent.
7. A plasma spray method in accordance with claim 1, characterized
in that process parameters are set so that a temperature of the
substrate (10, 110) amounts to between 250 and 850.degree. C.
during the carrying out of the method.
8. A plasma spray method in accordance with claim 7, characterized
in that process parameters are set so that a temperature of the
substrate (10, 110) is higher than a temperature boundary of in
particular 800.degree. C. only for a maximum period of time, in
particular of 5 minutes, during the carrying out of the method.
9. A plasma spray method in accordance with claim 1, characterized
in that the layer (11, 111), which forms the membrane, is composed
of a ceramic material which is an oxide of the perovskite type.
10. A plasma spray method in accordance with claim 9, characterized
in that the layer (11, 111) is composed of a perovskite which
includes lanthanum (La), strontium (Sr), cobalt (Co), iron (Fe),
chromium (Cr), titanium (Ta), barium (Ba), zirconium (Zr), cerium
(Ce), yttrium (Y), ytterbium (Yb), europium (Eu) or aluminum
(Al).
11. A plasma spray method in accordance with claim 1, characterized
in that the layer (11, 111) generated on the substrate (10, 110)
has a thickness of less than 150 micrometers.
12. A plasma spray method in accordance with claim 1, characterized
in that an overall flow rate of the process gas is smaller than 200
SLPM and in particular amounts to 100 to 160 SLPM.
13. A plasma spray method in accordance with claim 1, characterized
in that the process gas is a mixture of argon and helium.
14. A plasma spray method in accordance with claim 1, characterized
in that the process beam (2) is pivoted or rastered relative to the
coating surface (31, 131) of the substrate.
15. An ion conducting membrane, in particular a hydrogen ion
conducting membrane or an oxygen ion conducting membrane on a
substrate (10, 110) which is deposited as a layer (11, 111) on the
substrate (10, 110) with a plasma spray method in a process chamber
(12), wherein a starting material (P) is sprayed onto a coating
surface of the substrate (10, 110) by means of a process gas (G) in
the form of a process beam (2), wherein the starting material is
injected into a plasma at a low process pressure which is at most
10000 Pa and is partially or completely melted there, characterized
in that the substrate (10, 110) has pores (30) which are connected
amongst one another so that the substrate (10, 110) is gas
permeable and a portion of an overall pore surface of an overall
surface of the coating surface (31, 131) amounts to at least 40%.
Description
[0001] The invention relates to a plasma spray method for the
manufacture of an ion conducting membrane, in particular of a
hydrogen ion conducting membrane or of an oxygen ion conducting
membrane in accordance with the preamble of claim 1 and to an ion
conducting membrane, in particular to a hydrogen ion conducting
membrane or an oxygen ion conducting membrane in accordance with
the preamble of claim 15.
[0002] Ion conducting membranes are membranes which have a high
selective permeability for specific ions. Oxygen permeable
membranes are layers which have a high selective permeability for
oxygen or oxygen ions and are substantially impermeable for other
gases or ions. Correspondingly such membranes are used in order to
extract or to purify oxygen from gas mixtures or from fluid
mixtures. The same is true for hydrogen permeable membranes for the
extraction of hydrogen from gas mixtures or from fluid
mixtures.
[0003] Such membranes can be manufactured from different materials
they can, for example, be composed of complex oxide materials which
have a specific chemical composition and form specific phases. In
particular ceramic membranes are known which are composed of oxides
of the perovskite type and which are manufactured in the form of
thin, dense--this means non-porous--layers. Such membranes, for
example, have both an ion conductivity for oxygen or hydrogen and
also have an electron conductivity.
[0004] A material which is investigated and used today for the
manufacture, in particular of oxygen permeable membranes is a
ceramic material which has a perovskite structure and includes the
elements lanthanum (La), strontium (Sr), cobalt (Co) and iron (Fe)
besides oxygen. The substance is typically referred to as LSCF in
accordance with the respective first letter of these four
elements.
[0005] Oxygen permeable or hydrogen permeable membranes or
generally ion conducting membranes of such materials can, for
example, be manufactured by means of conventional manufacturing
techniques for ceramics, such as, for example, pressing, tape
casting, slip casting or sintering or also by means of thermal
spraying. In particular thermal spray processes are suitable for
the latter which are carried out in a vacuum, this typically means
that the spray process is carried out at a process pressure which
is smaller than the environmental pressure (normal air
pressure).
[0006] A thermal low pressure plasma spray process or a vacuum
plasma spray process is in particular suitable which is referred to
as LPPS method (low pressure plasma spraying). By means of this
vacuum plasma spray method particularly thin and dense layers can
be sprayed particularly well, i.e. such layers which are also
required for an ion conducting or oxygen permeable or hydrogen
permeable membrane.
[0007] In this connection the ion conducting membrane is deposited
as a layer on a substrate in a process chamber. For this purpose,
starting material is sprayed onto a surface of the substrate by
means of a process gas in the form of a process beam. The starting
material is injected into a plasma at a low process pressure which,
for example, is at most 10 000 Pa and is partially or completely
melted there.
[0008] During an LPPS method the ion conducting membrane is
deposited on a substrate in the form of a layer. The substrate
generally serves the purpose of supporting the thereby arising very
thin and brittle layer and to thus make it manageable.
[0009] For this reason, it is the object of the invention to
provide a plasma spray process with which a manageable combination
of substrate and ion conducting membrane, in particular a hydrogen
ion conducting membrane or an oxygen ion conducting membrane, can
be manufactured which moreover enables an effective extraction, in
particular of hydrogen or of oxygen from gas mixtures or from fluid
mixtures. It is furthermore the object of the invention to provide
a resistant combination of ion conducting membrane, in particular
of a hydrogen ion conducting membrane or of an oxygen ion
conducting membrane on a metallic porous substrate which moreover
enables an effective extraction, in particular of hydrogen or of
oxygen from gas mixtures or from fluid mixtures.
[0010] In accordance with the invention this object is satisfied by
a plasma spray method in accordance with claim 1 and by an ion
conducting membrane on a substrate in accordance with claim 15.
[0011] In accordance with the invention the substrate has pores
which are connected amongst one another so that the substrate is
gas permeable and a portion of an overall pore area of an overall
area of the coating surface amounts to at least 30%, in particular
to at least 40%, thus, for example, 40% or 45%. The sum of all
surfaces of the open pores of the coating surface of the substrate
should be understood to be the overall pore surface. The gas
mixture from which, in particular hydrogen or oxygen should be
extracted is guided to the open pores at the coating membrane and
comes into contact there with the membrane from a back substrate
side lying remote from the coating surface via the pores connected
amongst one another. The hydrogen or oxygen ions can then permeate
through the membrane starting from the open pores and thus hydrogen
or oxygen can be extracted from the gas mixture.
[0012] Thereby the substrate can also guide the fluid mixture or
the gas mixture from which the hydrogen or oxygen should be
extracted to the membrane in addition to supporting the membrane.
Furthermore, it is ensured by the mentioned lower boundary of the
overall pore surface that a large portion of the membrane also
comes into contact with the fluid mixture or the gas mixture and
thus that an effective extraction of hydrogen or oxygen is also
ensured.
[0013] The overall pore area can, for example, be determined
thereby that the pores of the coating surface are colored in and
the area of the colored in pores is determined by an optical
measurement process.
[0014] In an embodiment of the invention the pores have a mean pore
size of at least 1 micrometer, with this statement, in particular
not only relating to the pores at the substrate surface, but also
to the pores in the interior of the substrate. The mean pore size
is, in particular determined thereby that, as described above, the
open pores resulting from a cut through the substrate are colored
in. In order to determine the mean pore size a straight line is
subsequently placed onto the cut. Then the individual sections of
the mentioned line, which sections lie on the pores are measured
and a mean value is formed from the measured lengths. This mean
value corresponds to the mean pore size.
[0015] Investigations have shown that the through-flow is strongly
hindered which results in a worse transport of the gas mixture to
the membrane for pores having a pore size smaller than 1
micrometer.
[0016] In an embodiment of the invention the substrate has a useful
porosity of at least 20%, in particular of at least 30%, thus, for
example, 30% or 40% with respect to an overall volume of the
substrate. The useful porosity results from the quotient of a
hollow space volume connected amongst one another to the overall
volume of the substrate. The thus so-called closed hollow spaces
which have no connection to the environment of the substrate are
not of interest here. The open hollow spaces result in the pores at
the coating surfaces of the substrate via which pores the gas
mixture or fluid mixture can come into contact with the
membrane.
[0017] The gas mixture or the fluid mixture is particularly well
guided to the membrane which enables a particularly effective
extraction of hydrogen or of oxygen from gas mixtures or fluid
mixtures due to the use of a substrate having the mentioned
porosity.
[0018] An estimation of the porosity can also take place starting
from the abovementioned determination of the overall pore area. On
the assumption that the overall pore area is approximately equal in
each of the layers parallel to the coating surface the porosity can
thereby be calculated.
[0019] The useful porosity can, for example, be determined thereby
that it is determined which volume of a gas or a fluid the
substrate can take up. The useful porosity can then be determined
from the ratio of thus determined volume to the overall volume of
this substrate.
[0020] The substrate is in particular manufactured by means of a
sintering method. Thereby a free shaping of the substrate is
enabled and a particularly high porosity of the substrate can
furthermore be achieved.
[0021] In an embodiment of the invention micro-passages are
introduced into the substrate before or after the coating for the
improvement of a gas glow possibility in the direction of a coating
surface. The micro-passages in particular have a diameter of
between 5 and 150 micrometers and are typically introduced into the
substrate by means of a laser drilling method. Thereby a
particularly effective transport of the gas mixture or of the fluid
mixture to the membrane is possible.
[0022] The micro-passages are in particular orientated in the
direction of the coating surface from a back substrate side lying
remote form the coating surface and end before the coating surface.
Thereby, the gas mixture or fluid mixture which is guided via the
back substrate side is, on the one hand, very effectively guided to
the membrane. On the other hand, it is prevented through the ending
before the coating surface that too large recesses arise at the
coating surface which, as described above, negatively influence the
quality of the layer and thus of the membrane too strongly.
[0023] It is also possible that the micro-passages reach up to the
coating surface and therefore quasi form a pore. The introduction
then takes place, in particular prior to the coating so that the
micro-passages are covered by the membrane.
[0024] Since the membrane with the substrate should be used in
particular also in highly corrosive environments and at high
temperatures of above 500.degree. C., the iron alloy should include
chromium, whereby a high resistance to corrosion can be
achieved.
[0025] In an embodiment of the invention it is therefore suggested
that the substrate, on which the layer forming the membrane is
deposited, is manufactured from an iron alloy which has a chromium
portion which is larger than 20 weight percent and in particular
larger than 25 weight percent, i.e. for example, 22 or 30 weight
percent.
[0026] It is thereby achieved that the substrate and thus also the
combination of substrate and ion conducting membrane is very
resistant and in particular corrosion resistant. At the same time
the membrane manufactured by means of the plasma spray method has a
good ion conductivity, whereby an effective extraction of hydrogen
or oxygen from gas mixtures or from fluid mixtures is enabled.
[0027] Beside chromium the iron alloy can in particular also
include carbon. Furthermore, also further constituents such as, for
example, cobalt, manganese, molybdenum, niobium, vanadium or
tungsten can be present.
[0028] The substrate can, for example, be made of an alloy of 47%
nickel (Ni), 22% chromium (Cr), 18% iron (Fe), 9% molybdenum (Mo),
1.5% cobalt (Co), 0.6% tungsten (W), 0.1% carbon (C), 1% manganese
(Mn), 1% silicon (Si) and 0.008% boron (B).
[0029] The chromium included in the substrate can, however, lead to
problems on carrying out the method at too high temperatures of the
substrate. Chromium particles can then easily arrive in the coating
surface and react to chromium oxide there. Thereby a layer can
arise which hinders an ion exchange between gas and membrane.
Furthermore, chromium oxide cannot conduct any electrons. However,
excess electrons arise during use of the membranes which electrons
then have to be guided away from the substrate. In this respect, a
chromium oxide layer would likewise be of hindrance.
[0030] In the method in accordance with the invention relatively
high temperatures arise at the substrate due to the working
principle. This is true for the coating process itself, but is also
true for a heating phase of the substrate which takes place prior
to the actual coating. In this respect, a formation of the
described chromium oxide layer can be brought about. On the other
hand, however, also the danger is present that the structure of the
substrate is damaged by too high temperatures. These damages are
referred to as so-called "creeping".
[0031] In an embodiment of the invention process parameters are set
so that a temperature of the substrate amounts to between 250 and
850.degree. C. during the deposition of the layer. In this
connection in particular a process enthalpy, a spacing of the
substrate to a plasma torch and a period of time in which the
process beam is applied to the substrate without interruption or a
frequency of the application with the process beam are to be
understood as process parameters.
[0032] Dense layers can be generated in a comparatively short time
in the mentioned temperature range which is required for the use of
the layer as a membrane.
[0033] The temperature of the substrate can be measured during the
process by means of a pyrometer known per se. In this connection,
the heat radiation emitted by the substrate is measured and
evaluated. The measured temperatures can be used for a regulation
of the process parameters. However, it is also possible that the
temperatures are determined in a test phase and that the required
process parameters for maintaining the temperature boundaries are
determined in this test phase. The so determined process parameters
can then be used for a subsequent application of the method and the
temperature boundaries can thus be maintained.
[0034] The described damages of the substrate through "creeping"
and/or through the formation of chromium oxide are not only
dependent on the absolute temperature, but also on the duration of
how long the temperature is present.
[0035] For this reason, the process parameters are set, in
particular so that a temperature of the substrate is only higher
than the temperature boundary for a maximum period of time during
the carrying out of the method. The maximum temperature and the
maximum period of time are strongly dependent on the material of
the substrate. On the use of a metallic substrate, for example,
having the above-mentioned composition, the maximum temperature
amounts to in particular 800.degree. C. and the maximum period of
time in particular amounts to 5 minutes. Furthermore, the
above-mentioned temperature range of 250 to 850.degree. C. is still
true for the maximum temperature.
[0036] Preferably an inert atmosphere or an atmosphere with reduced
oxygen content is present during the spraying in the process
chamber.
[0037] The membrane preferably also has an electron conductivity
besides its ion conductivity.
[0038] The plasma spray process is preferably carried out so that
the plasma defocuses and accelerates the process beam. Particularly
good thin and dense layers can be manufactured by means of this
method.
[0039] In practice, it has been found advantageous when the process
pressure in the process chamber is set to a value of at least 50 Pa
and to at most 2000 Pa.
[0040] In a preferred embodiment, the layer forming the membrane is
composed of a ceramic material which is an oxide of the perovskite
type.
[0041] The method is in particular carried out so that the starting
material (precursor) is a powder whose chemical composition is
substantially the same as the chemical composition of the layer,
this means that a powder is used as a starting material which
substantially has the same chemical composition which the layer
sprayed should also have.
[0042] With regard to the oxygen permeability it has been
advantageously shown that the layer should be composed of a
perovskite which includes lanthanum (La), strontium (Sr), cobalt
(Co) and iron (Fe). In this connection the term "composed of" is
understood to mean that the substantial portion of the layer is
present in the form of a perovskite phase. Naturally, it is also
possible that also other phases are present to a lesser degree in
this layer.
[0043] For the manufacture of in particular hydrogen permeable
membranes, for example, a ceramic having a perovskite structure can
be used which includes the elements barium (Ba), zirconium (Zr),
cerium (Ce), yttrium (Y), ytterbium (Yb) and europium (Eu) or
lanthanium (La), strontium (Sr), chromium (Cr), yttrium (Y) and
aluminum (Al).
[0044] Preferably, the plasma spray method is carried out so that
the layer generated on the substrate has a thickness of less than
150 micrometers and preferably of 20 to 60 micrometers. This layer
thickness has been tried and tested for oxygen permeable membranes
or for hydrogen permeable membranes.
[0045] In practice it has been tried and tested that the overall
flow rate of the process gas on plasma spraying is smaller than 200
SLPM and in particular amounts to 100 to 160 SLPM (SLPM: standard
liter per minute).
[0046] For a first preferred embodiment of the method the process
gas is a mixture of argon and helium.
[0047] For a second preferred embodiment of the method the process
gas is composed of argon, helium and hydrogen.
[0048] It has also been found advantageous, when the process beam
is pivoted or rastered (scanned) relative to the surface of the
substrate. This can, for example, take place by the pivoting of the
plasma generator and/or of the plasma source and/or of the exit
nozzle. The process beam is thus guided relative to the substrate
so that the substrate is rastered, i.e. is covered once or a
plurality of times by the process beam. Alternatively, or in
addition, it is naturally also possible to move the substrate.
Naturally many possibilities are possible to realize this relative
movement between the process beam and the substrate. This pivot
movement and/or rastering of the substrate has the effect that the
oxygen introduced into the process chamber comes into contact with
the process beam or with the layer being formed on the substrate in
an as good as possible manner.
[0049] An ion conducting membrane on a metal porous substrate, in
particular an oxygen permeable membrane or a hydrogen permeable
membrane is further provided by the invention, which membrane is
deposited as a layer on the substrate by means of a plasma spray
method in a process chamber, wherein a starting material in the
form of a process beam is sprayed onto a surface of a substrate by
means of a process gas, with the starting material being injected
at a low process pressure, which is at most 10 000 Pa and is
partially or completely melted there and the substrate is
manufactured from a metal alloy which has a chromium proportion of
between 5 and 20 weight percent.
[0050] Further advantageous measures and preferred embodiments of
the invention result from the dependent claims.
[0051] In the following, the invention will be explained in detail
by means of embodiments and with reference to the drawings. In the
schematic drawing there is partially shown in section:
[0052] FIG. 1 a schematic illustration of an apparatus for carrying
out a method in accordance with the invention; and
[0053] FIG. 2 a schematic illustration of a substrate having a
layer deposited on the substrate;
[0054] FIG. 3 a detail of the substrate having the layer;
[0055] FIG. 4 a detail of the substrate in a top view; and
[0056] FIG. 5 a second embodiment of the substrate having a
layer.
[0057] The plasma spray method for the manufacture of an ion
conducting membrane in accordance with the invention will be
explained in the following with reference to the case of
application particularly relevant to practice, in which the
membrane is a permeable membrane selective for oxygen which thus
has an ion conductivity for oxygen. The membrane further preferably
also has an electron conductivity. The method is a thermal spray
method which is carried out in vacuum, thus at a process pressure
which is smaller than the environmental pressure.
[0058] FIG. 1 shows a plasma spray apparatus in a very schematic
illustration which is totally referred to with the reference
numeral 1 and is suitable for carrying out a method in accordance
with the invention. A metallic porous substrate 10 is furthermore
illustrated in FIG. 1 schematically, onto which an oxygen permeable
membrane in the form of layer 11 is deposited. A process chamber 12
is further indicated in which the method is carried out.
[0059] The method in accordance with the invention includes a
plasma spraying which is described in kind in WO-A-03/087422 or
also in U.S. Pat. No. 5,853,815. This plasma spray method is a
thermal spraying for the manufacture of a so-called LPPS thin film
(LPPS=Low Pressure Plasma Spraying).
[0060] A LPPS-based method is specifically carried out with the
plasma spray apparatus 1 illustrated in FIG. 1. During this a
conventional LPPS plasma spray method is changed from a process
point of view, wherein a space, which is through-flowed by plasma
("plasma flame" or "plasma beam"), is widened due to the change and
is widened to a length of up to 2.5 m. The geometric extent of the
plasma leads to a uniform widening--a "defocusing"--and to an
acceleration of a process beam which is injected into the plasma
with a feed gas. The material of the process beam which is
dispersed in the plasma to a cloud and is partially or completely
melted there arrives evenly distributed at the surface of the
substrate 10.
[0061] The plasma spray apparatus 1 illustrated in FIG. 1 includes
a plasma generator 3 known per se having a non-closer illustrated
plasma torch for the generation of a plasma. In a manner known per
se a process beam 2 is generated by the plasma generator 3 from a
starting material P, a process gas or a process gas mixture G and
electrical energy E. The introduction of these components E, G and
P is symbolized in FIG. 1 by the arrows 4, 5, 6. The generated
process beam 2 exits through an exit nozzle 7 and transports the
starting material P in the form of a process beam 2, in which the
material particles 21 are dispersed in the plasma. This transport
is symbolized by the arrow 24. The material particles 21 are
generally powder particles. The morphology of the layer 11
deposited on the substrate is dependent on the process parameters
and in particular on the starting material P, the process enthalpy
and the temperature of the substrate 10. The plasma generator 3
and/or the plasma torch are preferably pivotable with regard to the
substrate 10 as is indicated by the double arrow A in FIG. 1. Thus,
the process beam 2 can be moved to and fro over the substrate 10 in
a pivot movement.
[0062] It can be determined for how long and how frequently a
certain point on the substrate 10 is impinged by the process beam
without interruption by this to and fro movement. The longer this
period of time is or the more frequent this point is impinged, the
higher the temperature of the substrate 10 is at this point. The to
and fro movement, as well as the other process parameters are set
so that a temperature of the substrate 10 amounts to between 250
and 850.degree. C. The process parameters required for this are
determined in particular in test runs. Furthermore, the process
parameters are set so that the temperature of the substrate is only
larger than 800.degree. C. for at most 5 minutes. Thus, on the one
hand, the formation of a chromium oxide layer and, on the other
hand, so-called creeping are prevented or only enabled to a minimum
degree.
[0063] The porous substrate 10 is manufactured from an iron alloy
by sintering which has a chromium proportion which is larger than
25 weight percent. The substrate is composed, for example, of an
alloy composed of 47% nickel (Ni), 22% chromium (Cr), 18% iron
(Fe), 9% molybdenum (Mo), 1.5% cobalt (Co), 0.6% tungsten (W), 0.1%
carbon (C), 1% manganese (Mn), 1% silicon (Si) and 0.008% boron
(B).
[0064] The substrate has a useful porosity of 30% with respect to
an overall volume of the substrate.
[0065] In this connection the starting material P is injected into
a plasma defocusing the material beam and is partially or
completely melted therein or at least made plastic at a low process
pressure which is at most 10 000 Pa and preferably at least 50 Pa
and at most 2000 Pa for the described LPPS process. For this
purpose a plasma having a sufficiently high specific enthalpy is
generated so that a very thin and dense layer 11 arises on the
substrate. The variation of the structure is significantly
influenced and can be controlled by the coating conditions, in
particular the process enthalpy, the work pressure in the coating
chamber as well as the process beam. Therefore, the process beam 2
has properties which can be determined by the controllable process
parameters.
[0066] For the manufacture of the oxygen permeable membrane, the
layer 11 is generated so that it has a very dense
microstructure.
[0067] First of all the method step of generating the layer 11 by
means of LPPS will be explained in detail.
[0068] A powder having a suitable composition is selected as a
starting material P as will still be explained further on in
detail. As already mentioned the plasma flame is already very long
during the LPPS method due to the set process parameters in
comparison to conventional plasma spray. Furthermore, the plasma
flame is very strongly widened. A plasma with a high specific
enthalpy is generated, whereby a high plasma temperature results. A
very high introduction of energy into the material particles 21 is
brought about through the high enthalpy and the length and/or the
size of the plasma flame, which material particles 21 are thereby,
on the one hand, strongly accelerated and, on the other hand,
brought to a very high temperature so that they melt on very well
and are also very hot after their deposition on the substrate 10.
Since, on the other hand, the plasma flame and thus the process
beam 2 are very strongly widened, the local heat flow in the
substrate 10 is so low so that a thermal damage of the material is
avoided. It is in particular avoided that the porous structure of
the substrate 10 is damaged at the boundary to the layer 11 which
would influence the capability of use of the layer 10 as a
membrane. The widened plasma flame further has the result that,
typically on the one time covering of the substrate 10 with the
process beam 2, the material particles 21 are deposited in the form
of individual splats which do not yet generate a continuous layer,
this means a connected layer. Thereby very thin layers 11 can be
manufactured. The high kinetic and thermal energy which the
material particles obtain during their long stay in the plasma
flame in comparison to conventional plasma spray methods facilitate
the formation of a very dense layer 11 which in particular has few
boundary layer hollow spaces between splats lying on top of one
another.
[0069] The plasma is, for example, generated in a plasma torch
known per se in the plasma generator 3 having an electric direct
current and is generated by means of a pin cathode as well as a
ring-shaped anode. The power consumption of the plasma torch lies
in the region of up to 180 kW. The power supplied to the plasma,
the effective power, can be determined empirically with regard to
the resulting layer structure. The effective power which is
provided by the difference between the electric power and the heat
dissipated by cooling from experience lies, e.g. in the range of 40
to 130 kW, in particular 80 to 100 kW. For this purpose, it has
been tried and tested when the electric current for the generation
of the plasma lies between 1000 and 3000 A, in particular between
1500 and 2600 A.
[0070] A value of between 10 and 10000 Pa, preferably of between 50
and 2000 Pa is selected for the process pressure of the LPPS-TF
plasma spraying for the generation of the oxygen permeable membrane
in the process chamber 12.
[0071] The starting material P is injected into the plasma as
powder material.
[0072] The process gas for the generation of the plasma is
preferably a mixture of inert gases, in particular a mixture of
argon Ar, helium He and possibly hydrogen H. In practice, the
following gas flow rates for the process gas have been tried and
tested in particular:
[0073] Ar flow rate: 30 to 150 SLPM, in particular 50 to 100
SLPM;
[0074] H.sub.2 flow rate: zero to 20 SLPM, in particular 2 to 10
SLPM;
[0075] He flow rate: zero to 150 SLPM, in particular 20 to 100
SLPM,
[0076] with the overall flow rate of the process gas preferably
being smaller than 200 SLPM and in particular amounting to 100 to
160 SLPM.
[0077] It can be advantageous when the substrate is
moved--additionally or alternatively--during the material
deposition by means of rotational movement or pivotal movements
relative to this cloud.
[0078] In the following, reference is made to the example
particularly relevant for practice in which the oxygen permeable
membrane is composed of a ceramic which beside oxygen includes the
elements lanthanum (La), strontium (Sr), cobalt (Co) and iron (Fe).
Such ceramics are referred to as LSCF. In this connection, it is
driven for that the membrane should be composed as completely as
possible of a perovskite structure. However, it is naturally
understood that the invention is not limited to such substances,
but in particular it is also suitable for other ceramic materials,
specifically oxides of the perovskite type.
[0079] As already mentioned the starting material P is provided in
the form of a powder. The plasma spray method is then carried out
so that the chemical composition of the layer is at least
substantially the same as the chemical composition of the starting
material.
[0080] LSCF as a ceramic material belongs to the oxides of the
perovskite type which substantially have the form ABO.sub.3. In
this connection A represents La.sub.xSr.sub.1-x and B represents
Co.sub.yFe.sub.1-y. However, it should be noted that the
stoichiometry must not necessarily be satisfied exactly. It is
certainly possible that the La content and the Sr content and/or
the Co content and the Fe content do not exactly match to one. Also
the oxygen content can deviate from the precise stoichiometry. For
this reason, it is common to state the oxygen content with
3-.sigma., with .sigma. being the deviation of the oxygen content
from the stoichiometric equilibrium weight. The minus sign
indicates that this deviation is generally an oxygen deficiency,
this means that the oxygen is present under-stoichiometrically.
[0081] In the example described here LACF is present in the form of
La.sub.0.58Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3-.sigma.. The
starting material P is present as powder. Different methods can be
used for the manufacture of the powder particles: for example,
spray drying or a combination of melting and subsequent breaking
and/or grinding of the solidified melt.
[0082] The manufacture of such powders is known per se and does not
require a detailed explanation in this context. In view of the
plasma spraying it is preferred when the powder seeds, for example,
have a size of 25.+-.5 .mu.m.
[0083] The value of .sigma. for the deviation of the oxygen content
from this stoichiometry, for example, amounts to 0.3.
[0084] For the two examples described in the following,
La.sub.0.58Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3-.sigma. is
respectively used as the starting material. The process pressure in
the process chamber 12 is set to a value between 50 and 2000 Pa. A
plasma beam or a process beam 2 of high enthalpy is generated by
means of a plasma torch which can generate a plasma of high
specific enthalpy of up to 10,000 to 15,000 kJ/kg and which
consumes a power of up to 180 kW. The process beam 2 has a length
of 1000 to 2000 mm and a diameter of up to 200-400 mm. The length
of the process beam 2 substantially corresponds to the spray
distance, this means the distance D between the exit nozzle 7 and
the substrate 10. A porous plate of a high temperature nickel-based
alloy or of a refractory ceramic serves, for example, as a
substrate. The starting material P is introduced by means of two
powder supplies, with the feed rate being 120 g/min, typically
amounting to 40 g/min. By means of a pivot movement of the plasma
torch a thin and dense layer 11 is applied onto the substrate 10,
with the high introduction of energy in the material particles 21
and the high (ultrasonic) velocity in the process beam 2 enabling a
very dense structure of the layer 11. The layer 11 is sprayed up
until it has an overall thickness of 20-60 gm. The coating time
amounts to approximately one minute. During the thermal spraying
oxygen is supplied to the process chamber 12 and indeed having a
flow rate of at least 1%, preferably of at least 2% of the overall
flow rate of a process gas. Hereby the reduction and the
degradation of the starting material P or one of its components is
avoided or at least strongly reduced. The deposition or the
separation of elemental Co or Fe or their connections is avoided or
at least strongly reduced. From this, it results that the chemical
composition and the phase composition of the layer 11 is
substantially the same as that of the starting material P.
EXAMPLE 1
[0085] The process is carried out as is described above. The
mixture of argon and helium is used as a process gas, with the Ar
flow rate amounting to 80 SLPM and with the He flow rate amounting
to 40 SLPM, so that the overall flow rate of the process gas
amounts to 120 SLPM. The current for the generation of the plasma
amounts to 2600 A.
EXAMPLE 2
[0086] The process is carried out as is described above. A mixture
of argon, helium and hydrogen is used as a process gas, with the Ar
flow rate amounting to 80 SLPM, the He flow rate amounting to 20
SLPM and with the H.sub.2 flow rate amounting to 6 SLPM, so that
the overall flow rate of the process gas amounts to 106 SLPM. The
current for the generation of the plasma amounts to 2600 A.
[0087] In both cases oxygen permeable membranes result whose
chemical composition and perovskite phase structure is
substantially the same as that of the starting material.
[0088] A layer 11 acting as an oxygen permeable membrane on a
porous substrate 10 is very schematically illustrated in FIG. 2.
The substrate has pores 30 which are uniformly distributed in the
substrate 10 and are thus connected amongst one another so that the
substrate 10 is gas-permeable. The mentioned connections cannot be
seen in FIG. 2 since this is a very simplified illustration, on the
one hand, and, on the other hand, only is a section in a plane, the
pores are, however, naturally arranged in three dimensions. The
layer 11 is arranged at a coating surface 31 of the substrate 10. A
pressurized gas mixture contacts at a back substrate side 32 lying
opposite of the coating surface 31 from which mixture the oxygen
should be extracted. The gas mixture is symbolized by the arrow 33.
The gas mixture is transported to the coating surface 31 and thus
to the layer 11 via the pores 30 connected amongst one another. Due
to the permeability of the layer 11 for oxygen ions these exit
through the layer 11 and finally combine to oxygen molecules
O.sub.2. Thereby the oxygen is extracted from the gas mixture.
[0089] A section of the substrate 10 and the layer 11 is shown in a
detailed view in FIG. 3. In this connection pores 30 open to the
layer 11 are illustrated which likewise are connected to other
pores. The pores 30 are formed between metal particles 34 which are
combined by sintering. It can clearly be seen in FIG. 3 that the
gas mixture can be guided to the layer 11 through the substrate
10.
[0090] In FIG. 4 a very schematic section of the substrate is
illustrated in a top view. The pores 30 again form between the
metal particles 34. A portion of an overall pore surface of an
overall surface of the coating surface in this connection amounts
to approximately 40%, this means that the portion of the nonhatched
area to the overall surface of the illustrated rectangle amounts to
approximately 40%. The mean pore size of the pores 30 in this
connection amounts to at least 1 micrometer.
[0091] In FIG. 5 a substrate 110 having a layer 111 is illustrated.
In order to improve the supply of the gas mixture to the layer 111
the substrate 110 has micro-passages 140 which can have a diameter
of between 5 and 150 micrometers. The micro-passages 140 reach from
a back substrate side 132 in the direction of a coating surface
131, wherein they end prior to arriving at the coating surface 131.
The micro-passages 140 are introduced by laser drilling either
before or after the coating.
* * * * *