U.S. patent application number 11/281056 was filed with the patent office on 2007-05-17 for sensing element and method of making the same.
Invention is credited to Fenglian Chang, Rick D. Kerr, Kerry J. Kruske.
Application Number | 20070108047 11/281056 |
Document ID | / |
Family ID | 38039625 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070108047 |
Kind Code |
A1 |
Chang; Fenglian ; et
al. |
May 17, 2007 |
Sensing element and method of making the same
Abstract
Disclosed herein is a sensing element comprising: an
electrochemical cell; wherein the sensing element comprises a metal
selected from the group consisting of Pd and alloys and
combinations comprising at least one of the foregoing; and wherein
the electrically conductive element is thermally stable at
temperatures of greater than or equal to about 1,200.degree. C.
Inventors: |
Chang; Fenglian; (Grand
Blanc, MI) ; Kruske; Kerry J.; (Oakey, MI) ;
Kerr; Rick D.; (Fenton, MI) |
Correspondence
Address: |
Paul L. Marshall;Delphi Technologies, Inc.
M/C 480-410-202
P.O. Box 5052
Troy
MI
48007
US
|
Family ID: |
38039625 |
Appl. No.: |
11/281056 |
Filed: |
November 16, 2005 |
Current U.S.
Class: |
204/400 |
Current CPC
Class: |
G01N 27/4075 20130101;
G01N 27/4071 20130101 |
Class at
Publication: |
204/400 |
International
Class: |
G01N 27/26 20060101
G01N027/26 |
Claims
1. A sensing element, comprising: a sensing electrode and a
reference electrode in physical contact with an electrolyte; and an
electrically conductive element, wherein the electrically
conductive element comprises a component selected from the group
consisting of a ground plane, a lead, a via, a contact pad, a
resistor, and combinations comprising at least one of the foregoing
components; wherein the electrically conductive element comprises
irregularly shaped Pd.
2. The sensor element of claim 1, wherein the electrically
conductive element comprises greater than or equal to about 50 wt.
% Pd, based upon the total weight of the electrically conductive
element.
3. The sensing element of claim 1, wherein the electrically
conductive element comprises about 0.5 wt. % to about 25 wt. % of
the metal oxide, based the total weight of the electrically
conductive element.
4. The sensing element of claim 1, wherein the metal oxide is
selected from the group consisting of zirconia, yttria, alumina,
lanthana, ceria, magnesia, scandia, and combinations comprising at
least one of the foregoing.
5. The sensing element of claim 1, wherein the electrically
conductive element comprises a sheet resistivity of less than or
equal to about 6.times.10.sup.-3 .mu..OMEGA./cm.
6. The sensing element of claim 5, wherein the sheet resistivity is
less than or equal to about 4.times.10.sup.-3 .mu..OMEGA./cm.
7. The sensing element of claim 6, wherein the sheet resistivity is
less than or equal to about 3.times.10.sup.-3 .mu..OMEGA./cm.
8. The sensing element of claim 1, wherein the electrically
conductive element is thermally stable at temperatures of about
1,200 to about 1,530.degree. C.
9. The sensing element of claim 1, wherein the Pd has a surface
area of about 0.5 m.sup.2/g to about 5.0 m.sup.2/g.
10. The sensing element of claim 9, wherein the surface area is
about 1 m.sup.2/g to about 4.0 m.sup.2/g.
11. The sensing element of claim 10, wherein the surface area is
about 2.0 m.sup.2/g to about 3.0 m.sup.2/g.
12. The sensing element of claim 1, wherein the electrically
conductive element further comprises about 0.25 wt % to about 20 wt
% of a metal selected from the group consisting of Pt, Rh, Ir, as
well as combinations comprising at least one of the foregoing
metals, wherein the weight is based upon a total weight of the
electrically conductive element.
13. The sensing element of claim 12, wherein the electrically
conductive element comprises about 0.25 wt % to about 10 wt % of
metal.
14. The sensing element of claim 12, wherein the metal is Rh.
15. The sensing element of claim 12, wherein the metal and the Pd
are a solid solution.
16. A sensing element, comprising: a sensing electrode and a
reference electrode in physical contact with an electrolyte; and an
electrically conductive element, wherein the electrically
conductive element comprises a component selected from the group
consisting of a ground plane, a lead, a via, a contact pad, a
resistor, and combinations comprising at least one of the foregoing
components; wherein the electrically conductive element comprises
Pd having a surface area of about 0.5 m.sup.2/g to about 5.0
m.sup.2/g.
17. A method of making a sensing element, comprising: disposing a
sensing electrode and a reference electrode in physical contact
with an electrolyte; forming a conductive element precursor
material comprising Pd having a surface area of about 0.5 m.sup.2/g
to about 5.0 m.sup.2/g; disposing the precursor material on a
supporting surface to define an electrically conductive element;
and heating the precursor material to a temperature of greater than
or equal to about 1,450.degree. C. for a sufficient period of time
to sinter the precursor material and form the sensing element.
18. The method of claim 17, wherein the electrically conductive
element is a sensing lead in physical contact with the sensing
element, and further comprising disposing additional precursor
material in physical contact with the reference electrode to form a
reference lead.
Description
TECHNICAL FIELD
[0001] The present disclosure is related to a sensing element and a
method of making and, in particular, to a sensing element
containing a palladium and/or palladium alloy electrically
conductive element and a method of making the same.
BACKGROUND
[0002] Sensors, in particular gas sensors, have been utilized for
many years in several industries (e.g., flues in factories, in
furnaces and in other enclosures; in exhaust streams such as flues,
exhaust conduits, and the like; and in other areas). For example,
the automotive industry has used exhaust gas sensors in automotive
vehicles to sense the composition of exhaust gases, namely, oxygen.
A sensor may be used to determine the exhaust gas content for
alteration and optimization of the air to fuel ratio for
combustion.
[0003] One type of sensor employs an ionically conductive solid
electrolyte between porous electrodes. For oxygen detection, solid
electrolyte sensors are used to measure oxygen activity differences
between an unknown gas sample and a known gas sample. In the
application of a sensor for automotive exhaust, the unknown gas is
exhaust and the known gas, i.e., reference gas, is usually
atmospheric air because the oxygen content in air is relatively
constant and readily accessible. This type of sensor is based on an
electrochemical galvanic cell operating in a potentiometric mode to
detect the relative amounts of oxygen present in an automobile
engine's exhaust. When opposite surfaces of this galvanic cell are
exposed to different oxygen partial pressures, an electromotive
force (EMF) is developed between the electrodes according to the
Nernst equation.
[0004] According to the Nernst principle, chemical energy is
converted into electromotive force. Thus, a gas sensor based upon
this principle typically consists of an ionically conductive solid
electrolyte material, a porous electrode with a porous protective
overcoat exposed to exhaust gases ("sensing electrode"), and a
porous electrode exposed to the partial pressure of a known gas
("reference electrode"). Sensors used for automotive applications
typically employ a yttria stabilized zirconia based electrochemical
galvanic cell with porous platinum electrodes, operating in
potentiometric mode, to detect the relative amounts of a particular
gas, such as oxygen for example, that is present in an automobile
engine's exhaust. Also, a typical sensor has a ceramic heater
attached to help maintain the sensor's ionic conductivity at low
exhaust temperatures. When opposite surfaces of the galvanic cell
are exposed to different oxygen partial pressures, an electromotive
force is developed between the electrodes on the opposite surfaces
of the zirconia wall, according to the Nernst equation: E = ( - R
.times. .times. T 4 .times. .times. F ) .times. ln .function. ( P O
2 ref P O 2 ) ##EQU1##
[0005] where: [0006] E=electromotive force [0007] R=universal gas
constant [0008] F=Faraday constant [0009] T=absolute temperature of
the gas [0010] P.sub.O.sub.2.sup.ref=oxygen partial pressure of the
reference gas [0011] P.sub.O.sub.2=oxygen partial pressure of the
exhaust gas
[0012] Due to the large difference in oxygen partial pressure
between fuel rich and fuel lean exhaust conditions, the
electromotive force (EMF) changes sharply at the stoichiometric
point, giving rise to the characteristic switching behavior of
these sensors. Consequently, these potentiometric oxygen sensors
indicate qualitatively whether the engine is operating in fuel rich
or fuel lean conditions, without quantifying the actual air to fuel
ratio of the exhaust mixture.
[0013] In addition to oxygen, the exhaust gas contains many
components including carbon monoxide, carbon dioxide, hydrogen,
water, nitrogen oxides, nitrogen, and a variety of hydrocarbons and
hydrocarbon derivatives. Because the exhaust gas is a
non-equilibrium mixture containing products of incomplete
combustion, the oxygen partial pressure is not an equilibrium
pressure. Because the oxygen partial pressure is not at
equilibrium, sensors do not operate at stoichiometric air to fuel
ratios per the Nernst equation. In addition, the use of
zirconia-based electrolyte materials contributes to non-ideal
sensor behavior.
[0014] To provide a means of monitoring the cell potential and to
circumvent at least some of the difficulties associated with
non-equilibrium conditions, catalytic electrodes are used to both
catalyze the oxidation reactions and to equilibrate the local
oxygen concentrations. Ideal sensors produce a sharp EMF or voltage
step at a stoichiometric air to fuel ratio per the Nernst equation.
Manufactured sensors, however, exhibit non-ideal behaviors, for
example, a broadened voltage transition that occurs over a range of
air to fuel ratios near the stoichiometric ratio. In addition, the
sensor EMF may depend upon mass transport processes, adsorption,
desorption and chemical reactions that occur at the electrodes.
[0015] Platinum (Pt) is widely used as the material for various
electrically conductive components of exhaust sensors such as, for
example, electrodes, sensing elements, heaters, ground planes,
leads, vias, contact pads, and the like. The use of Pt in exhaust
sensors such as oxygen, nitrogen oxide and ammonia is desirable
because it can withstand many process application temperatures
without degradation due to its exceptional physical and chemical
properties.
[0016] Accordingly, a need exists in the sensor manufacturing art
for less expensive methods and/or materials for producing such
sensors.
SUMMARY
[0017] Disclosed herein is a sensing element comprising: an
electrochemical cell; wherein the sensing element comprises a metal
selected from the group consisting of Pd and alloys and
combinations comprising at least one of the foregoing; and wherein
the electrically conductive element is thermally stable at
temperatures of greater than or equal to about 1,200.degree. C.
[0018] Also disclosed herein is a method of making a sensing
element, comprising: forming a precursor material comprising a
plurality of irregularly shaped Pd particles and an organic
vehicle; disposing the precursor material on a supporting surface
to define an electrically conductive element; and heating the
precursor material to a temperature of greater than or equal to
about 1450.degree. C. for a sufficient period of time to sinter the
precursor material and form the sensing element; wherein the
electrically conductive element is thermally stable at a
temperature of about 1,200.degree. C.
[0019] The above described and other features are exemplified by
the following figures and detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Refer now to the figures, which are exemplary embodiments,
and wherein like elements are numbered alike.
[0021] FIG. 1 is an expanded isometric view of an oxygen sensing
element.
[0022] FIG. 2A shows a scanning electron microscope (SEM) image of
an irregularly shaped Pd powder.
[0023] FIG. 2B shows a SEM image of a spherically shaped,
relatively high surface area Pd powder.
[0024] FIG. 3A is a copy of an optical micrograph showing cracks
and delamination in a Pd thick film electrically conductive element
formed from a relatively high surface area Pd powder, after firing
at about 1,530.degree. C.
[0025] FIG. 3B is a copy of an optical micrograph showing a Pd
thick film electrically conductive element formed from a relatively
low surface area Pd powder, after firing at about 1,530.degree.
C.
[0026] FIG. 4A is a copy of an optical micrograph of a Pd--Rh alloy
thick film electrically conductive element formed from a relatively
high surface area Pd powder, after firing at about 1,530.degree.
C.
[0027] FIG. 4B is a copy of an optical micrograph of a Pd--Rh alloy
thick film electrically conductive element formed from a relatively
low surface area Pd powder with spherically shaped particles, after
firing at about 1,530.degree. C.
[0028] FIG. 4C is a copy of an optical micrograph of a Pd--Rh alloy
thick film electrically conductive element formed from a relatively
low surface area Pd powder with irregularly shaped particles, after
firing at about 1,530.degree. C.
[0029] FIG. 5 is a graphical representation of the thermal
stability of exposed Pd--Rh alloy electrically conductive elements
as a function of rhodium (Rh) concentration.
[0030] FIG. 6 is a graphical representation of the thermal
stability of embedded Pd--Rh alloy electrically conductive elements
as a function of rhodium (Rh) concentration.
DESCRIPTION
[0031] At the outset of the description, it should be noted that
the terms "first," "second," and the like, herein do not denote any
order, quantity, or importance, but rather are used to distinguish
one element from another, and the terms "a" and "an" herein do not
denote a limitation of quantity, but rather denote the presence of
at least one of the referenced items. 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). It
is noted that the terms "bottom" and "top" are used herein, unless
otherwise noted, merely for convenience of description, and are not
limited to any one position or spatial orientation. Furthermore,
all ranges disclosed herein are inclusive and combinable (e.g.,
ranges of "up to about 25 weight percent (wt. %), with about 5 wt.
% to about 20 wt. % desired, and about 10 wt. % to about 15 wt. %
more desired," are inclusive of the endpoints and all intermediate
values of the ranges, e.g., "about 5 wt. % to about 25 wt. %, about
5 wt. % to about 15 wt. %", etc.). Finally, unless defined
otherwise, technical and scientific terms used herein have the same
meaning as is commonly understood by one of skill in the art to
which this invention belongs. The suffix "(s)" as used herein is
intended to include both the singular and the plural of the term
that it modifies, thereby including one or more of that term (e.g.,
the metal(s) includes one or more metals).
[0032] Disclosed herein is a sensing element comprising
electrically conductive element(s) (e.g., ground plane(s), lead(s),
via(s), contact pad(s), low conductance resistor(s) (e.g., a
heater)) comprising palladium (Pd), e.g., comprising Pd in
combination (e.g., solid solution, alloy, and/or mixture) with
rhodium (Rh), iridium (Ir), and/or platinum (Pt), and combinations
comprising at least one of the foregoing. These electrically
conductive elements are thermally stable at temperatures comparable
to and in some instances greater than the thermal stability of
platinum, which is unexpected because Pd has a relatively low
melting point. For example, the electrically conductive elements
are thermally stable at temperatures of greater than or equal to
about 1,200.degree. C., e.g., temperatures of about 1,250.degree.
C. to about 1,530.degree. C., which spherical Pd materials may not
be capable of withstanding. Optionally, a metal oxide can be
included in the electrically conductive element(s). In addition,
the sheet resistivity of these electrically conductive element(s)
can be controlled, e.g., can be varied by varying the concentration
of the metal oxide, and/or of the Rh, Ir and/or Pt in the
element.
[0033] An exemplary planar oxygen-sensing element 10 is shown in
FIG. 1. Although described herein in connection with an
oxygen-sensing element, it is to be understood that the
electrically conductive element(s) disclosed herein can be utilized
in other types of sensors such as temperature sensors and gas
sensors (e.g., nitrogen sensors, hydrogen sensors, hydrocarbon
sensors, ammonia sensors, and the like). In addition, although
described in connection with a planar sensing element, it is to be
understood that the electrically conductive element(s) can be
employed in other types of sensing elements such as, for example,
wide-range, switch-type, conical, and the like.
[0034] As shown in FIG. 1, sensing element 10 can comprise a
sensing end 10s and a terminal end 10t. The sensing element 10 can
comprise a sensing (i.e., first, exhaust gas, or outer) electrode
12, a reference gas (i.e., second or inner) electrode 14, and an
electrolyte portion 16. The electrolyte portion 16 can be disposed
at the sensing end 10s with the electrodes 12, 14 disposed on
opposite sides of, and in ionic contact with, the electrolyte
portion 16, thereby creating an electrochemical cell
(12/16/14).
[0035] A reference gas channel 18 can be disposed on the side of
the reference electrode 14 opposite the electrolyte portion 16. The
reference gas channel 18 can be disposed in fluid communication
with the reference electrode 14 and with a reference gas (e.g., the
ambient atmosphere, the exhaust gas, or another gas supply).
[0036] A heater 20 can be disposed on a side of the reference gas
channel 18 opposite the reference electrode 14, for maintaining
sensing element 10, and in particular, the sensing end 10s of the
sensing element, at a desired operating temperature. The heater 20
can be any heater capable of maintaining the sensor end 10s at a
sufficient temperature to facilitate the various electrochemical
reactions therein. The heater 20 can be, for example, Pt, aluminum
(Al), Pd, and the like, as well as oxides, mixtures, and alloys
comprising at least one of the foregoing metals. Optionally, the
heater can be one of the electrically conductive element(s). The
heater 20 can be disposed on one of the support layers by various
methods such as, for example, screen-printing. The thickness of the
heater 20 can be about 5 micrometers to about 50 micrometers, or
so.
[0037] A protective layer L1 can be disposed adjacent to the
sensing electrode 12 opposite the electrolyte portion 16. The
protective layer L1 can comprise a solid portion 24 and a porous
portion 22 disposed adjacent to the sensing electrode 12. The
porous portion 22 can be a material that enables fluid
communication between the sensing electrode 12 and the gas to be
sensed. For example, the porous portion 22 can comprise a porous
ceramic material formed from a precursor comprising a ceramic
(e.g., spinel, alumina, zirconia, and/or the like), a fugitive
material (e.g., carbon black), and an organic binder. The fugitive
material can provide pore formation in the fired layer. The porous
portion 22 can be formed, for example, from a precursor comprising
about 70 to about 80 weight percent (wt. %) of one or more of the
foregoing ceramic materials, about 5 to about 10 wt. % of the
fugitive material, and about 15 wt. % to about 20 wt. % of an
organic binder, based upon the total weight of the precursor, which
can be applied using various methods including thick film methods,
and the like, followed by sintering.
[0038] In order to further protect the sensing electrode 12, a
protective coating 26 can optionally be disposed over the porous
portion 22 and optionally over layer L1. As with the porous portion
22, at least in the area of the porous portion 22, the protective
coating 26 allows fluid communication between the sensing electrode
12 and the gas to be sensed. Possible materials for the protective
coating 26 can comprise spinel, alumina (e.g., stabilized alumina),
and other protective coatings employed in sensors.
[0039] If desired, one or more support layers can be disposed on a
side of the sensing electrode 12 opposite the electrolyte 16;
between the reference gas channel 18 and the heater 20, and on a
side of the heater 20 opposite the reference gas channel 18. As
shown, insulating layer L1 is disposed on a side of the sensing
electrode 12 opposite the electrolyte portion 16; support layers
L3-L6 are disposed between the reference electrode 14 and the
heater 20; and support layer L7 is disposed on a side of the heater
20 opposite the reference gas channel 18. A support layer L2 can be
employed with the electrolyte 16 disposed therethrough, attached to
an end thereof, or the electrolyte can comprise the entire
layer.
[0040] The support layers, e.g., L2-L7, that can provide structural
integrity (e.g., protect various portions of the gas sensor from
abrasion and/or vibration, and the like, and provide physical
strength to the sensor); physically separate and electrically
isolate various components; and provide support for various
components that can be formed in or on the layers. Depending on the
arrangement, the support layers can each comprise the same or
different materials, e.g., a dielectric material (e.g., alumina
(Al.sub.2O.sub.3)), an electrolytic material (e.g., zirconium oxide
(zirconia)), protective material, and the like. Each of the support
layers can comprise a thickness of up to about 500 micrometers so,
depending upon the number of layers employed, or, more
particularly, about 50 micrometers to about 200 micrometers.
Although illustrated herein as comprising seven layers L1-L7, it
should be understood that the number of layers could be varied
depending on a variety of factors.
[0041] Electrolyte portion 16 can comprise a solid electrolyte. The
electrolyte portion 16 can be disposed through layer L2 in a
variety of arrangements. For example, the electrolyte portion 16
can be attached to L2 at the sensing end such that the electrolyte
portion 16 forms the sensing end of L2, disposed in an aperture
(not illustrated) adjacent to the sensing end 10s, and disposed in
an opening through the layer L2. The latter arrangement eliminates
the use of excess electrolyte. Any shape can be used for the
electrolyte, with the size and geometry of the various inserts, and
therefore the corresponding openings, being dependent upon the
desired size and geometry of the adjacent electrodes. The openings,
inserts, and electrodes can comprise a substantially compatible
geometry such that sufficient exhaust gas access to the
electrode(s) is enabled and sufficient ionic transfer through the
electrolyte is established to attain the desired sensor function.
The electrolyte can comprise a thickness of up to about 500
micrometers or so, more specifically, about 25 micrometers to about
500 micrometers, and even more specifically, about 50 micrometers
to about 200 micrometers.
[0042] The electrolyte 16 can be, for example, any material that is
capable of permitting the electrochemical transfer of oxygen ions
while inhibiting the passage of exhaust gases, desirably has an
ionic/total conductivity ratio of approximately unity, and is
compatible with the environment in which the sensor will be
utilized. Possible electrolyte materials can comprise any material
capable of functioning as a sensor electrolyte including, but not
limited to, zirconium oxide (zirconia), cerium oxide (ceria),
calcium oxide, yttrium oxide (yttria), lanthanum oxide, magnesium
oxide, ytterbium (III) oxide (Yb.sub.2O.sub.3), scandium oxide
(Sc.sub.2O.sub.3), and so forth, as well as combinations comprising
at least one of the foregoing. If zirconia is employed, it can be
stabilized with, for example, with calcium, barium, yttrium,
magnesium, aluminum, lanthanum, cesium, gadolinium, and so forth,
as well as combinations comprising at least one of the foregoing
materials. For example, the electrolyte can be alumina stabilized
zirconia and/or yttrium stabilized zirconia.
[0043] Accordingly, formation of electrically conductive element(s)
of the sensing element 10, can comprise preparing a suitable
precursor material such as an ink, paste, slurry and/or the like.
For example, an precursor (ink) can be formed by mixing a metal
powder with a sufficient quantity of an organic vehicle to attain
the desired adhesion to the substrate after firing, as well as
other properties. It is noted that the Pd materials described
herein can be used as the sensing electrode to sense hydrogen, but
are not particularly sensitive to oxygen. Hence, although it is
contemplated that the Pd materials can be employed to form the
electrodes, they are generally employed only to form the other
electrically conductive components (e.g., leads, heater, ground
plane, vias, and so forth).
[0044] The metal powder can comprise a Pd that can optionally be
combined with Rh, Ir, and/or Pt. The Pd powder can comprise a
surface area of about 0.5 m.sup.2/g to about 5.0 m.sup.2/g, more
particularly, about 1 m.sup.2/g to about 4.0 m.sup.2/g, and more
particularly still, about 2.0 m.sup.2/g to about 3.0 m.sup.2/g. The
Pd powder also can comprise a particle size distribution at 90
percent (i.e., a P.S.D. 90) of about 1 micrometer to about 5
micrometers, and a tap density of about 0.5 grams per cubic
centimeter (g/cm.sup.3) to about 4.0 g/cm.sup.3.
[0045] Optionally, the precursor material can comprise a metal
oxide, for example, to improve the adhesion of the electrically
conductive element(s) to underlying substrate (where applicable),
and/or impart beneficial properties such as inhibition of further
sintering. Possible metal oxides can comprise ceria, lanthana,
magnesia, zirconia, yttria, alumina, scandia, and the like, and
mixtures comprising at least one of the foregoing. The amount of
metal oxide employed is dependent upon the particular metals
employed and the temperatures used in forming the sensor. The
amount of metal oxide can be up to about 25 wt %, based upon a
total amount of solids in the precursor material. In some
embodiments, the amount of metal oxide can be about 2 wt % to about
10 wt % (e.g., for a Pd material), while in other embodiments, the
amount of metal oxide may be about 0.2 wt % to about 2 wt % (e.g.,
for a Pd--Rh material).
[0046] The metal powder and optional metal oxide can be combined
with a vehicle (e.g., an organic vehicle) to enable deposition of
the precursor onto the desired portion(s) of the sensor
element.
[0047] It has been discovered, unexpectedly, that electrically
conductive element(s) formed from a relatively low surface area
(e.g., less than or equal to about 5.0 m.sup.2/g) and irregularly
shaped (e.g., a sponge or a flake) Pd powder (as shown in FIG. 2A),
in comparison to those formed from spherically shaped Pd powder (as
shown in FIG. 2B), can provide increased thermal stability. It
should be understood that relatively high surface area powders have
a spherical shape. Not to be bound by any theory, it is thought
that the irregularly shaped Pd particles can result in anisotropic
densification of the thick film during high temperature firing
(e.g., greater than or equal to about 1,450.degree. C.), with
in-plane densification being reduced. In contrast, it is thought
that the spherically shaped Pd particles can yield isotropic
densification of the thick film, with large three-dimensional
shrinkages, which can cause electrically conductive element
cracking and delamination of the electrically conductive element
from the substrate during high temperature firing. Thus, the Pd
powder can comprise an irregular shape, and the Rh, Pt and/or Ir
powders can comprise either an irregular or a spherical shape.
[0048] After mixing the metal powder, the organic vehicle, and the
optional metal oxide, the ink can comprise about 60 wt. % to about
70 wt. % solids and about 30 wt % to about 40 wt. % of the organic
vehicle. The ink can comprise less than or equal to about 20 wt. %
of each of the Rh, Ir, and/or Pt, more particularly about 0.5 wt. %
to about 15 wt. %, and more particularly still about 2 wt % to
about 10 wt %, based on the total weight of the metals in the
precursor, with the balance comprising Pd. The precursor can
comprise about 0.5 wt. % to about 25 wt. % metal oxide, based on
the total concentration of solids in the precursor, more
particularly about 0.5 wt. % to about 20 wt. %, and more
particularly still, about 0.7 wt. % to about 18 wt. %.
[0049] Once prepared, the conductive element precursor material can
be applied to the desired area of the sensor, using various
application technique(s) such as thick film technique(s) including
screen printing, painting, spraying, dipping, coating, and the
like. Depending upon the particular electrically conductive
element, as well as the particular technique employed, optional
thickener(s), binder(s), additive(s), fugitive material(s) (e.g.,
carbon, insoluble organic material, and the like), and so forth
(hereinafter additive(s)), can be employed in the precursor
material in an amount of less than or equal to about 40 wt. %
additives for screen printing, less than or equal to about 60 wt. %
additives for pad flexing (painting), less than or equal to about
75 wt. % additives for spray coatings, less than or equal to about
90 wt. % additives for dip coatings, based on the total weight of
thick film inks. Possible additives include: 1-ethoxypropan-2-ol,
turpentine, squeegee medium, 1-methoxy-2-propanol acetate, butyl
acetate, dibutyl phthalate, fatty acids, acrylic resin, ethyl
cellulose, pine oil, 3-hydroxy, 2,2,4-trimethylpentyl isobutyrate,
terpineol, butyl carbitol acetate, cetyl alcohol, cellulose
ethylether resin, and so forth, as well as combinations comprising
at least one of the foregoing.
[0050] The thickness of the electrically conductive elements (e.g.,
the leads, the heater, the ground plane, the contact pads,
temperature sensor, the vias, and other electrically conductive
components) is dependent upon the particular element. The thickness
can be up to the thickness of the layer or so (e.g., for a via),
or, more particularly, about 1 micrometers (.mu.m) to about 50
micrometers, or, even more particularly, about 3 micrometers to
about 35 micrometers, and still more particularly about 7
micrometers to about 25 micrometers.
[0051] Furthermore, the element precursor material can be applied
during any point during the manufacturing process; i.e., before the
substrate is fired (green), before the substrate is fully fired
(bisque), or after the substrate is fully fired. In each case, once
the element precursor material has been applied, the substrate is
heated to a temperature sufficient to sinter the precursor material
(e.g., greater than or equal to about 1450.degree. C. for about 2
hours). Optionally, the electrically conductive elements can be
co-fired with green layers (alumina (Al.sub.2O.sub.3), zirconia
(ZrO.sub.2), and so forth). For a coating comprising a metal oxide
such as zirconia, alumina, for example, temperatures of about
1,400.degree. C. or greater can be employed.
[0052] The foregoing sensor, and others comprising more or less
cells, can be formed using a variety of methods in which the
components can be formed and fired separately or formed (optionally
laminated), and co-fired. For example, an electrolyte tape can be
formed and partially fired to the bisque state. The precursor
material can be prepared as described above and deposited on the
appropriate portions of the support layer(s) and/or the electrolyte
tape and connecting electrical leads to the ink. A protective layer
and support layer(s) can be disposed accordingly, with a ground
plane, temperature sensor, and/or heater disposed therein as
desired. The lay-up can then be heated to a sufficient temperature
to volatilize the organics and to sinter the metals in the
precursor, thereby forming the sensor.
[0053] In one embodiment, during use, the sensor can be disposed in
a fluid to be sensed, e.g., an exhaust stream. Based upon the
condition of the fluid to be sensed, i.e. rich or lean, oxygen can
be pumped in or out of the sensor by the pumping cell. The
increase/decrease, accordingly, creates an oxygen partial pressure
difference between the oxygen at the sensing electrode and at the
reference electrode, thereby developing an electromotive force that
can be correlated with the oxygen concentration.
[0054] The following examples are merely to further illustrate the
sensor element, and/or the electrically conductive element(s), and
are not intended to limit the scope thereof.
EXAMPLES
Example 1
Effect of the Surface Area of the Pd Powder
[0055] Electrically conductive layers were formed from Pd powder
and 8 wt % alumina (based upon the total weight of the solids), and
were fired at a temperature of about 1,530.degree. C.
[0056] FIG. 3A shows an optical micrograph of a Pd electrically
conductive layer formed from a relatively high surface area (13.5
square meters per gram (m.sup.2/g)) Pd powder, after firing at
1,530.degree. C. for 2 hours. As shown, the surface of the
electrically conductive layer was cracked and portions were
delaminated.
[0057] FIG. 3B shows an optical micrograph of a Pd electrically
conductive layer formed from a relatively low surface area (1.9
m.sup.2/g) Pd powder having an irregular shape, after firing at
1,530.degree. C. for 2 hours. As shown, surface of the electrically
conductive layer was continuous and uniform, and the adhesion of
the electrically conductive layer to the ceramic substrate was
maintained.
[0058] The results show that the physical properties of the Pd
powder used in the ink determine, at least in part, the
electrically conductive layer morphology after firing. The use of
low surface area, irregularly shaped Pd powder can be advantageous
for relatively high temperature firing applications (e.g., greater
than 1,400.degree. C.).
Example 2
Effect of the Physical Properties of the Pd Powder in Pd Alloys
[0059] Electrically conductive layers were formed using 5 wt. % Rh,
and 0.7 wt. % alumina, balance Pd. Various Pd powders were used.
All of the electrically conductive layers were fired at a
temperature of about 1,530.degree. C. for 2 hours.
[0060] FIG. 4A shows an optical micrograph of a Pd--Rh alloy
electrically conductive layer formed from the relatively high
surface area (12.8 m.sup.2/g) Pd powder. FIG. 4B shows an optical
micrograph of a Pd--Rh alloy electrically conductive layer formed
from the relatively low surface area (1.9 m.sup.2/g) Pd powder.
FIG. 4C shows an optical micrograph of a Pd--Rh alloy electrically
conductive layer formed from a relatively low surface area (1.9
m.sup.2/g) Pd powder having an irregular shape, after firing at
1,530.degree. C. The result show that the physical property of the
Pd powder in Pd--Rh electrically conductive layers were similar to
the non-alloyed Pd electrically conductive layers illustrated in
FIGS. 3A and 3B. The Pd--Rh electrically conductive layers formed
from relatively low surface area, irregular-shaped Pd powder
provided a continuous, uniform, crack-free, lift-free electrically
conductive layers.
[0061] In addition, the adhesion of the Pd--Rh electrically
conductive layer was comparable to that of the pure Pd electrically
conductive layer. The results show that the electrically conductive
layers formed from the Pd--Rh have good adhesion and morphology
with lower oxide loading (e.g., 0.7 wt %), which is advantageous in
some instances because the resistivity of the electrically
conductive layer increases with the concentration of metal oxide.
Not to be bound by any theory, it is believed that various finely
dispersed Pd--Rh internal oxides (e.g., oxides formed as part of a
Pd and Rh solid solution) were formed during firing, which promoted
adhesion between the electrically conductive layer and substrate,
and reduced the loading requirement of oxides in the precursor.
Example 3
Sheet Resistivity of Pd--Rh Electrically Conductive Layers
[0062] A Veeco FPP-100 four-point probe instrument was used to
measure the sheet resistivity of fired Pd--Rh alloy electrically
conductive layers. The sheet resistivity of a standard Pt
electrically conductive layer was used as a reference, as shown
below in Table 1. TABLE-US-00001 TABLE 1 Electrically conductive
layer Sheet composition after firing (wt. %) Firing Resistivity
Metal Temperature (.times.10.sup.-3 Sample Pt Pd Rh Oxide (.degree.
C.) .mu..OMEGA./cm) A 92.0 N/A N/A 8.0 1,508 6.7 B N/A 96.3 3.0 0.7
1,504 3 C N/A 94.3 5.0 0.7 1,505 4.8 D N/A 89.4 9.9 0.7 1,505 10.5
E N/A 94.4 4.9 0.7 1,493 12 .mu..OMEGA./cm = micro Ohm per
centimeter
[0063] As shown in Table 1, sheet resistivity values for the Pd--Rh
alloy electrically conductive layers were above or below that of
the Pt electrically conductive layer (A), based on the
concentration of the Rh and/or the metal oxide. Although Pd
containing electrically conductive layers can have a sheet
resistivity of up to 15.times.10.sup.-3 .mu..OMEGA./cm, sheet
resistivities of less than or equal to about 12.0.times.10.sup.-3
.mu..OMEGA./cm, or, more specifically, less than or equal to about
6.times.10.sup.-3 .mu..OMEGA./cm, or, even more specifically, less
than or equal to about 4.times.10.sup.-3 .mu..OMEGA./cm, and even
more specifically, less than or equal to about 3.times.10.sup.-3
.mu..OMEGA./cm.
[0064] The results show that the Pd and Pd containing materials
(e.g., Pd--Rh solid solution, alloy, so forth) thick film inks can
be used to form both conductors such as ground, leads, vias, and
contact pads, as well as resistors of controlled, lower
conductance, such as heaters in sensor applications, each of which
has a different requirement for electrical resistivity. For
example, leads and vias require low resistance to minimize energy
loss. Therefore, Samples (B) and (C) were good candidates for
forming applications such as leads and vias in sensor elements. In
contrast, the requirement for a ground plane is less strict; a
ground plane electrically conductive layer can have marginal
electrical conductivity after firing. Therefore, Samples (D) and
(E) were good candidates for forming ground planes. Heaters can
have high resistance in order to generate power. Therefore, by
adjusting the electrically conductive layer thickness, all of the
foregoing compositions can be used to make heaters.
Example 4
Weight Change of Exposed Electrically Conductive Layers as a
Function of Temperature
[0065] The change in weight of an electrically conductive layer
material is an indicator of its thermal stability: a weight gain
can indicate that the electrically conductive layer has been
oxidized, and a weight loss can indicate that an oxide has been
decomposed and that some material has been volatilized. The
relative measure of weight change (gain or loss) is considered a
measure of the thermal stability of the electrically conductive
layer.
[0066] FIG. 5 is a graphical representation of the weight change of
various exposed electrically conductive layers (e.g., Pd--Rh alloys
vs Pt) as a function of temperature, wherein the numbers represent
the weight percent of each component (e.g., 3Rh97Pd is 3 wt % Rh
and 97 wt % Pd). As shown, all of the electrically conductive layer
compositions showed a reduction in weight at temperatures of
greater than or equal to about 1,200.degree. C. As shown in FIG. 5,
the weight change of the Pd--Rh alloy electrically conductive
layers was comparable to that of the Pt electrically conductive
layers in an exposed environment at temperatures below about
1,100.degree. C. At temperatures of greater than or equal to about
1,100.degree. C., the Pd--Rh alloy electrically conductive layers
showed some weight gain, e.g., due to oxidation, whereas the weight
of the Pt electrically conductive layer remained unchanged. At a
temperature of about 1,350.degree. C., both the Pd--Rh alloy
electrically conductive layers and the Pt electrically conductive
layer showed a decrease in weight, e.g., as a result of metal
volatilization.
[0067] Thus, the thermal stability of exposed Pd--Rh electrically
conductive layers was comparable to the Pt electrically conductive
layer at temperatures less than or equal to about 1,100.degree. C.
In addition, increasing the concentration of Rh in the Pd--Rh alloy
electrically conductive layers increased the thermal stability of
the Pd--Rh alloy electrically conductive layers at high
temperature.
Example 5
Thermal Stability of Embedded Electrically Conductive Layers
[0068] In an exhaust sensor application, the electrically
conductive layers are mostly embedded. The weight change as a
function of temperature was compared for embedded electrically
conductive layers having the same compositions as those used in
Example 4. The Pt electrically conductive layer contained about 8
wt. % metal oxide.
[0069] FIG. 6 is a graphical representation of the weight change of
the embedded electrically conductive layers as a function of
temperature. As shown in FIG. 6, the weight change of the Pd--Rh
electrically conductive layers was comparable to the Pt
electrically conductive layer at temperatures of less than or equal
to about 900.degree. C. At temperatures of about 900.degree. C. to
about 1,200.degree. C., the weight of all of the electrically
conductive layers increased, e.g., due to high temperature
oxidization. At temperatures greater than 1,300.degree. C., the
Pd--Rh electrically conductive layers showed a reduction in weight,
whereas the weight of the Pt electrically conductive layer remained
relatively stable. However, as is shown by the scale employed, the
reduction in weight was not significant. Embedded Pd--Rh
electrically conductive layers were thermally stable at a
temperature of up to about 1,300.degree. C. Additionally, Pd
containing electrically conductive elements are stable at
temperatures of 700.degree. C. to 900.degree. C. (i.e.,
temperatures at which sensors are often employed).
[0070] In summary, the Pd and Pd containing electrically conductive
element ink compositions: 1) can be printed using thick film
techniques; 2) can provide a lower sheet resistivity than Pt
electrically conductive elements, allowing them to be used as leads
(e.g., as shown in Example 2); 3) can be used for various
electrically conductive elements such as heaters, leads, ground
planes, vias, contact pads, and so forth; 4) can provide thermal
stability at temperatures of greater than 1,200.degree. C., in both
exposed and embedded environments; 5) can provide thermal
stability, especially in embedded environments; 6) can provide a
significant cost reduction in comparison to other electrically
conductive element materials such as Pt electrically conductive
elements; 7) can provide good adhesion to underlying substrate (as
is evident from the ability to bond to the substrate with lower and
no metal oxide loading); 8) can be used to replace any of the
electrically conductive elements other than sensing and reference
electrically conductive elements 12 and 14 without compromising the
physical, chemical and electric functionality of the sensor;
thereby reducing costs.
[0071] While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustration only, and such illustrations and
embodiments as have been disclosed herein are not to be construed
as limiting to the claims.
* * * * *