U.S. patent application number 09/739548 was filed with the patent office on 2002-08-01 for low-density coating for gas sensors.
Invention is credited to Kikuchi, Paul, Wadu-Mesthrige, Kaplia, Wu, Ming-Cheng.
Application Number | 20020102349 09/739548 |
Document ID | / |
Family ID | 24972807 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020102349 |
Kind Code |
A1 |
Wu, Ming-Cheng ; et
al. |
August 1, 2002 |
LOW-DENSITY COATING FOR GAS SENSORS
Abstract
A method for making a sensor is disclosed, comprising mixing a
metal oxide with a polymer to create a composition. The composition
is applied to at least a portion of the sensing element comprising
two electrodes with an electrolyte disposed therebetween, and
calcined to form a protective coating. A gas sensor created in
accordance with the above-referenced method is also disclosed.
Inventors: |
Wu, Ming-Cheng; (Rochester
Hills, MI) ; Wadu-Mesthrige, Kaplia; (Southfield,
MI) ; Kikuchi, Paul; (Fenton, MI) |
Correspondence
Address: |
VINCENT A. CICHOSZ
DELPHI TECHNOLOGIES, INC.
Mail Code: 480-414-420
P.O. Box 5052
Troy
MI
48007-5052
US
|
Family ID: |
24972807 |
Appl. No.: |
09/739548 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
427/126.1 ;
204/426; 427/126.2; 427/126.3 |
Current CPC
Class: |
G01N 27/4077
20130101 |
Class at
Publication: |
427/126.1 ;
427/126.2; 427/126.3; 204/426 |
International
Class: |
B05D 005/00; B05D
005/12; G01N 027/409 |
Claims
Claims:
1. A method for making a sensor, comprising: mixing a metal oxide
with a polymer to create a composition; applying said composition
to at least a portion of a sensing element comprising two
electrodes with an electrolyte disposed therebetween; and calcining
said sensing element.
2. The method of claim 1, wherein said metal oxide temperature
stability of up to about 900.degree. C. or greater and a surface
area of about 50 m.sup.2/g or greater.
3. The method of claim 2, wherein said metal oxide is selected from
the group consisting of alumina, silica, and mixtures comprising at
least one of the foregoing metal oxides.
4. The method of claim 3, wherein said alumina is selected from the
group consisting of theta-alumina, alpha-alumina, lanthanum oxide
stabilized theta-alumina, strontium oxide stabilized alumina,
barium oxide stabilized alumina, and combinations comprising at
least one of the foregoing metal oxides.
5. The method of claim 1, wherein said polymer is a material
selected from the group consisting of polystyrenes,
polymethylmethacrylates, polystyrene-divinylbenzenes and
combinations comprising at least one of the foregoing polymers.
6. The method of claim 1, wherein said polymer has a particle size
of about 0.2 .mu.m to about 100 .mu.m.
7. The method of claim 6, wherein said polymer has a particle size
of about 0.5 .mu.m to about 10 .mu.m.
8. The method of claim 1, wherein said polymer has a range of
particle sizes from about 0.2 .mu.m to about 100 .mu.m.
9. The method of claim 1, wherein said composition comprises about
3 wt. % to about 15 wt. % of said polymer.
10. The method of claim 9, wherein said composition comprises about
5 wt. % to about 10 wt. % of said polymer.
11. The method of claim 1, wherein said protective coating has a
thickness of up to about 300 .mu.m.
12. The method of claim 10, wherein said protective coating has a
thickness of about 100 .mu.m to about 200 .mu.m.
13. A gas sensor created according to the method of claim 1.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to gas sensors, and
particularly a low-density coating for the sensing element.
BACKGROUND
[0002] The automotive industry has used exhaust gas sensors in
automotive vehicles for many years to sense the composition of
exhaust gases, namely, oxygen. For example, a sensor is used to
determine the exhaust gas content for alteration and optimization
of the air to fuel ratio for combustion.
[0003] One type of sensor uses an ionically conductive solid
electrolyte between porous electrodes. For oxygen, solid
electrolyte sensors are used to measure oxygen activity differences
between an unknown gas sample and a known gas sample. In the use 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] With the Nernst principle, chemical energy is converted into
electromotive force. 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 ("exhaust gas electrode"), and a porous
electrode exposed to a known gas' partial pressure ("reference
electrode"). Sensors typically used in automotive applications use
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: 1 E = ( -
RT 4 F ) ln ( P O 2 ref P O 2 )
[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..sub.2.sup.ref=oxygen partial pressure of the
reference gas
[0011] P.sub.O.sub..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 fuel-rich or
fuel-lean, conditions without quantifying the actual air-to-fuel
ratio of the exhaust mixture.
[0013] In a conventional sensor, the sensor comprises a first
electrode capable of sensing an exhaust gas and a second electrode
capable of sensing a reference gas with an ionically conductive
solid electrolyte disposed therebetween. High temperatures and
materials such as silicon, lead and the like, present in engine
exhaust, can poison or otherwise damage the sensing electrode. In
order to prevent poisoning/damage to the sensing electrode, a
protective layer made of spinel or the like, has conventionally
been applied to the sensing electrode.
[0014] The protective layer is designed to allow for the electrodes
to sense the particular gas without inhibiting the performance of
the sensor. A thick layer (or multiple layers) of protective
coating more effectively inhibits the transmission of the poisoning
materials, but at the expense of a decrease in the efficiency of
the sensor. Furthermore, the protective layer itself can become
clogged, inhibiting passage of exhaust gases for sensing. One
conventional poison resistance technique comprises applying
multiple layers of a heat resistant metal oxide to the electrode to
form a protective layer. However, the multiple layers have a
tendency to change the performance of the sensor and only provide
limited poison protection.
[0015] Accordingly, there exists a need in the art for improved
protective coatings for gas sensors.
SUMMARY
[0016] The drawbacks and disadvantages of the prior art are
overcome by the low-density coating for a gas sensor and method for
making the same. The method comprises mixing a metal oxide with a
polymer to create a composition. The composition is applied to at
least a portion of the sensing element comprising two electrodes
with an electrolyte disposed therebetween, and calcined to form a
protective coating. A gas sensor created according to the
above-referenced method is also disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Referring now to the figures, which are meant to be
exemplary, not limiting, and wherein like elements are numbered
alike in several figures:
[0018] FIG. 1 is an expanded view of one embodiment of an oxygen
sensor.
[0019] FIG. 2 is graph showing a severe zinc-phosphorous poisoning
rapid aging test (RAT) for various sensors with hours of RAT
exposure time on the X axis (hours) and lean voltage on the Y axis
in millivolts (mv).
[0020] FIG. 3 is a graph showing a severe RAT test for various
sensors with hours of RAT exposure time on the X axis (hours) and
rich to lean response time on the Y axis in milliseconds (ms).
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] A protective coating for sensors, in particular oxygen
sensors, is formed from a composition comprising a metal oxide and
a fugitive material. Although described in connection with an
oxygen sensor, it is to be understood that the protective coating
can be employed with any type of sensor such as a nitrogen oxide
sensor, hydrogen sensor, hydrocarbon sensor, or the like. Although
described in connection with a planar sensor, it is to be
understood that the protective coating can be employed with any
type of sensor such as a conical, wide-range, and the like.
[0022] Referring to FIG. 1, the sensor element 10 is illustrated.
The exhaust gas (or outer) electrode 20 and the reference gas (or
inner) electrode 22 are disposed on opposite sides of, and adjacent
to, a solid electrolyte layer 30 creating an electrochemical cell
(20/30/22). On the side of the exhaust gas electrode 20 opposite
solid electrolyte 30 is a protective insulating layer 40 having a
dense section 44 and a porous section 42 that enables fluid
communication between the exhaust gas electrode 20 and the exhaust
gas. Meanwhile, disposed on the side of the reference electrode 22
opposites solid electrolyte 30 is a reference gas channel 60, which
is in fluid communication with the reference electrode 22 and
optionally with the ambient atmosphere and/or the exhaust gas.
Disposed on a side of the reference gas channel 60 opposite the
reference electrode 22 is a heater 62 for maintaining sensor
element 10 at the desired operating temperature. Typically disposed
between the reference gas channel 60 and the heater 62, as well as
on a side of the heater opposite the reference gas channel 60, are
one or more insulating layers 50, 52.
[0023] In addition to the above sensor components, conventional
components can be employed, including but not limited to, lead
gettering layer(s), leads, contact pads, ground plane(s), support
layer(s), additional electrochemical cell(s), and the like. The
leads (not shown), which supply current to the heater and
electrodes, are typically formed on the same layer as the
heater/electrode to which they are in electrical communication and
extend from the heater/electrode to the terminal end of the gas
sensor where they are in electrical communication with the
corresponding via (not shown) and appropriate contact pads (not
shown).
[0024] Insulating layers 50, 52, and protective layer 40, 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), and physically separate and
electrically isolate various components. The insulating layer(s),
which can be formed using ceramic tape casting methods or other
methods such as plasma spray deposition techniques, screen
printing, stenciling and others conventionally used in the art, can
each be up to about 200 microns (.mu.m) thick or so, with a
thickness of about 50 .mu.m to about 200 .mu.m preferred. Since the
materials employed in the manufacture of gas sensors preferably
comprise substantially similar coefficients of thermal expansion,
shrinkage characteristics, and chemical compatibility in order to
minimize, if not eliminate, delamination and other processing
problems, the particular material, alloy or mixture chosen for the
insulating and protective layers is dependent upon the specific
electrolyte employed. Typically these insulating layers comprise a
dielectric material such as alumina, and the like.
[0025] Disposed between the insulating layers 50, 52, is a heater
62 that is employed to maintain the sensor element at the desired
operating temperature. Heater 62 can be any conventional heater
capable of maintaining the sensor end at a sufficient temperature
to facilitate the various electrochemical reactions therein. The
heater 62, which is typically platinum, aluminum, palladium, and
the like, as well as mixtures, oxides, and alloys comprising at
least one of the foregoing metals, or any other conventional
heater, is generally screen printed or otherwise disposed onto a
substrate to a thickness of about 5 .mu.m to about 50 .mu.m.
[0026] The heater maintains the electrochemical cell (electrodes
20, 22 and electrolyte 30) at a desired operating temperature. The
electrolyte layer 30 can be solid or porous, can comprise the
entire layer or a portion thereof, can be any material that is
capable of permitting the electrochemical transfer of oxygen ions,
should have an ionic/total conductivity ratio of approximately
unity, and should be compatible with the environment in which the
gas sensor will be utilized (e.g., up to about 1,000.degree. C.).
Possible electrolyte materials can comprise any material
conventionally employed as sensor electrolytes, including, but not
limited to, zirconia which may optionally be stabilized with
calcium, barium, yttrium, magnesium, aluminum, lanthanum, cesium,
gadolinium, and the like, as well as oxides, alloys, and
combinations comprising at least one of the foregoing materials.
For example, the electrolyte can be alumina and/or yttrium
stabilized zirconia. Typically, the electrolyte, which can be
formed via many conventional processes (e.g., die pressing, roll
compaction, stenciling and screen printing, tape casting
techniques, and the like), has a thickness of up to about 500 .mu.m
or so, with a thickness of about 25 .mu.m to about 500 .mu.m
preferred, and a thickness of about 50 .mu.m to about 200 .mu.m
especially preferred.
[0027] It should be noted that the electrolyte layer 30 and porous
section 42 can comprise an entire layer or a portion thereof; e.g.,
they can form the layer, be attached to the layer (porous
section/electrolyte abutting dielectric material), or disposed in
an opening in the layer (porous section/electrolyte can be an
insert in an opening in a dielectric material layer). The latter
arrangement eliminates the use of excess electrolyte and protective
material, and reduces the size of gas sensor by eliminating layers.
Any shape can be used for the electrolyte and porous section, 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. It is preferred that the
openings, inserts, and electrodes have 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.
[0028] The electrodes 20, 22, are disposed in ionic contact with
the electrolyte layer 30. Conventional electrodes can comprise any
catalyst capable of ionizing oxygen, including, but not limited to,
platinum, palladium, osmium, rhodium, iridium, gold, ruthenium,
zirconium, yttrium, cerium, calcium, aluminum, silicon, and the
like, and oxides, mixtures, and alloys comprising at least one of
the foregoing catalysts. As with the electrolyte, the electrodes
20, 22 can be formed using conventional techniques. Some possible
techniques include sputtering, painting, chemical vapor deposition,
screen printing, and stenciling, among others. If a co-firing
process is employed for the formation of the sensor, screen
printing the electrodes onto appropriate tapes is preferred due to
simplicity, economy, and compatibility with the co-fired process.
Electrode leads and vias (not shown) in the insulating and/or
electrolyte layers are typically formed simultaneously with
electrodes.
[0029] An alternative sensor design can include a conical sensor.
The conical sensor typically comprises an electrolyte body, having
an inner surface, an outer surface, a cavity opening and a cavity
terminus located at opposing ends of electrolyte body. An inner
electrode is disposed on the inner surface, and an outer electrode
is disposed on outer surface. The conical sensor can be formed in
any generally cylindrical shape and is preferably tapered from the
cavity opening to the cavity terminus. A protrusion is typically
formed on the sensor element at a point between the cavity opening
and the cavity terminus to define an upper shoulder and a lower
shoulder that preferably extends completely around the
circumference of a cross-section of electrolyte body. The
protrusion is generally configured and dimensioned to engage a
surface within a shell portion of the gas sensing apparatus into
which the sensor element is received, thereby causing the inactive
portion of the sensor, e.g., the portion above and including the
lower shoulder, to extend out of the shell portion while the active
portion extends into the shell portion to contact the exhaust gas.
The materials, as indicated above for the planar sensor, can also
be utilized with the conical sensor.
[0030] Following the formation of the sensing element 10, a
protective coating can be applied to the sensing element 10. This
protective coating may optionally be used to coat the entire
sensing element 10 or a portion of the sensing element 10.
Conventional protective coatings are formed of a composition
comprising a metal oxide and a fugitive material. Metal oxides
having an affinity to filter out materials such as silica and zinc
phosphate compounds, and other poisons, as well as having a high
temperature stability (e.g., up to about 900.degree. C. or so), and
preferably having a high surface area (e.g., a surface area of
about 50 m.sup.2/g or greater, with about 100 m.sup.2/g or greater
preferred). Some possible metal oxides can include alumina, silica,
and the like, and mixtures comprising at least one of the foregoing
metal oxides. Conventional fugitive materials include carbon-based
materials, such as carbon black. As used herein, a "fugitive
material" means a material that will occupy space until the
electrode is fired, thus leaving porosity in the coating.
[0031] Although protective coatings formed using carbon black are
suitable, they fail to obtain a desired density. Essentially,
carbon black particles have a size of about 0.02 .mu.m to about 0.2
.mu.m. Aggregates, which may be as large as 1.0 .mu.m typically
break during coating preparation.
[0032] In contrast, when a polymer is employed as the fugitive
material, a greater non-breakable particle size range is available.
A suitable polymer can be a polymer material including, but not
limited to, polystyrene, poly(methylmethacrylate),
polystyrene-divinylbenzene, and the like, and combinations
comprising at least one of the foregoing materials. The average
size of the polymer particles can be up to about 100 .mu.m, with
about 0.2 .mu.m to about 50 .mu.m preferred, about 0.5 .mu.m to
about 10 .mu.m more preferred, and about 0.5 .mu.m to about 5 .mu.m
especially preferred. Most preferably, the polymer comprises a
range of particle sizes of about 0.2 .mu.m to about 100 .mu.m.
[0033] When forming the protective coating, the metal oxide
component is preferably prepared by forming a slurry. The metal
oxide component of the protective coating can be prepared by mixing
a coarse (e.g., about 30 .mu.m or greater in diameter, with about
30 .mu.m to about 50 .mu.m in diameter, preferred), high-surface
area (e.g., about 100 m.sup.2/g or greater) alumina, such as
theta-alumina (.theta.-Al.sub.2O.sub.3), lanthanum oxide
(La.sub.2O.sub.3) stabilized .theta.-Al.sub.2O.sub.3, strontium
oxide stabilized alumina, barium oxide stabilized alumina, with a
fine (e.g., about 10 .mu.m or less in diameter, with about 0.5
.mu.m or less in diameter preferred) alpha alumina
(.alpha.-Al.sub.2O.sub.3) and a binder, such as aluminum nitrate
(Al(NO.sub.3).sub.3) to form a base slurry. The slurry can comprise
about 45 weight percent (wt. %) to about 49 wt. % of
.theta.-Al.sub.2O.sub.3, about 45 wt. % to about 49 wt. % of
/.alpha.-Al.sub.2O.sub.3, and about 2 wt. % to about 10 wt. % of
Al(NO.sub.3).sub.3. In another embodiment, a slurry can be formed
of La.sub.2O.sub.3 stabilized alumina with about 47 wt. % of
La.sub.2O.sub.3 stabilized .theta.-Al.sub.2O.sub.3, about 47 wt. %
of .alpha.-Al.sub.2O.sub.3, and about 6 wt. % of
Al(NO.sub.3).sub.3. The percentage of solids in the slurry is about
45 wt. % to about 55 wt. %, with about 48 wt. % to about 52 wt. %
preferred. The slurry is stirred thoroughly prior to being milled
(e.g., using a vibro-energy grinding mill) for about 2 hours, or
so, to break down the aggregates of .theta.-Al.sub.2O.sub.3. During
milling, the size of the .theta.-Al.sub.2O.sub.3 aggregates
(d.sub.50) decrease to less than about 5 .mu.m.
[0034] Following the milling of this slurry, about 25 wt. % to
about 40 wt. %, with about 30 wt. % to about 35 wt. % preferred, of
the total solids of coarse La.sub.2O.sub.3 stabilized
.theta.-Al.sub.2O.sub.3 is mixed into the base slurry. The polymer
is then added to the slurry. About 3 wt. % to about 15 wt. %, with
about 5 wt. % to 10 wt. % preferred, is added to the base slurry to
obtain a low-density "fluffy" alumina slurry.
[0035] The slurry can then applied as a protective coating to at
least a portion of the sensing element. The sensing element is
immersed in the slurry, which is preferably stirred at a constant
speed and then withdrawn from the slurry. The amount of coating
deposited on the sensing element depends upon the physical and
chemical properties of the slurry, such as viscosity and pH, as
well as the withdrawal rate. For example, using a conical oxygen
sensor element, about 150 milligrams (mg) to about 350 mg of
protective coating adhered to the element (via wet pickup) by
manipulating the withdrawal rate. The protective coating created
was uniform and crack-free. About 200 mg to about 300 mg of wet
pickup (or about 120 mg to about 190 mg of calcined pickup) is
preferred.
[0036] Following the depositing of the coating on the sensing
element, it is optionally dried at temperatures up to about
100.degree. C. Next, the element can be calcined at a temperature
sufficient to bum off the fugitive material, such as about
500.degree. C. for about 2 hours, prior to assembly into the
sensor.
[0037] As with the pore size and porosity, the thickness of the
protective coating is based upon the ability to filter out
poisoning particulates while allowing passage of the exhaust gases
to be sensed. Although a multi-layered coating can be employed, the
protective coating is preferably a single layer having an overall
thickness of up to about 300 .mu.m, with a thickness of about 150
.mu.m to about 200 .mu.m preferred.
[0038] An experiment was completed with an oxygen sensor, having a
low-density coating, in a working engine. In this experiment, a
low-density alumina coating was prepared using unmilled, coarse
alumina and a fugitive polymer. The monodispersed polystyrene was
used as the fugitive material that has a particle size of about 0.5
.mu.m. The sensor was subjected to a 400-hour severe
zinc-phosphorous poisoning rapid aging test (RAT), which simulates
200,000 miles of vehicle age. FIGS. 2 and 3 illustrate engine
performance data of lean voltage and rich-to-lean transition time,
respectively. The results indicate that all of the oxygen sensor
parts coated with the low-density coating, formed from a
composition having about 5 wt. % to about 12 wt. % of a polymer,
passed 400 hours of severe RAT poisoning without noticeable
performance degradation. However, engine performance data from the
oxygen sensors parts having carbon black formed similar low-density
alumina coatings exhibited performance degradation after a 400-hour
RAT durability test.
[0039] The use of a polymer as a fugitive material produces a
low-density coating which inhibits the formation of glass that
covers coating surfaces after prolonged exposure to engine exhaust.
The polymer provides a consistent low-density alumina coating, has
a greater particle size range than conventional fugitive materials,
is chemically stable, and is physically non-breakable in slurry
preparation.
[0040] 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 apparatus and method have 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.
[0041] What is claimed is:
* * * * *