Note: Descriptions are shown in the official language in which they were submitted.
11~92~0
REFERENCE TO RELATED APPLICATION
Reference is made herein to our earlier Canadian
Patent Application Serial No. 319,639, filed January 15, 1979.
Background of the Invention
As discussed in our earlier application, oxygen gas
sensors containing solid electrolyte oxygen gas sensor
elements are used to measure the oxygen content of an
automotive exhaust gas for the purpose of regulating the
eficiency of the engine through control of the air to fuel
1~ ratio. These generally thimble-shaped sensor elements
having an inner conductive catalyst electrode on the inner
surface of the thimble and an outer conductive catalyst
electrode on the outer surface of the thimble are
conductively connected to a monitoring and actuating system
to adjust said air-fuel ratio.
In our earlier application, the use of a chemical
treatment, wherein the inner electrode was contacted with
an inorganic acid or acid salt, produced a chemical
treatment of the inner electrode and resulted in an
increased voltage output in the positive range for the
sensor element and also reduced the internal resistance of
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the solid electrolyte sensor element, both of which are beneficial to
the operation of the sensor. Also, as discussed thereinJ
when the chemically activated sensor elements are also
subiected to a current treatment, wherein the sensor
element is subjected to a direct current, with the outer
catalytic electrode as a cathode, and at an elevated
temperature and in the presence of a reducing gas, the
above properties are further enhanced and, in addition, the
switching response time required for swi~ching from rich to
lean gas composition readings is reduced.
In using the combined chemical and current
activation treatment of our previous application, however,
the need for the presence of a reducing gas at the outer
electrode during current activation was p.resent, as well as
the need for a recovery period during which the sensor
element was maintained at the elevated temperature, in
order to provide a stable condition within the solid
electrolyte body.
We have now discovered that if current activation
of the sensor element is combined with the chemical
activation, where the current activation is carried out by
applying a direct current to the sensor element, with the
outer electrode as an anode, only a nonoxidizing gas need be
present and, in addition, the need for a recovery period is removed,
where short time periods of current application are used.
Summar of the Invention
Y
An activated oxygen gas sensor element having an
increased voltage output under rich gas conditions,
shortened switching response time and reduced internal
resistance, where the element comprises a solid electrolyte
body, such as zirconium dioxide, having an inner conductive
catalyst electrode on the inner surface and an outer
conductive catalyst electrode on the outer surface, is
produced by contacting the inner conductive catalyst
electrode with an inorganic acid or acid salt and by
applying a direct current to the sensor element, with the
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outer electrode as an anode, with the current application effected while the
outer electrode is in the presence of a nonoxidizing atmosphere and at a
temperature in excess of 450C, the current density thereof being at least
5 milliamperes per square centimeter of the planar surface of the outer
conductive catalyst electrode.
DETAILED DESCRIPTION
The gas sensor elements that are subjected to the present prccess
to improve the properties thereof are generally in the shape of a closed
tubuIar me~ber, thimble-like, with the sensor body formed of a solid
electrolyte material, such as stabilized zirconium dioxide. mis general
shape of the electrolyte body is known in the æ t, as well as the solid
electrolyte usable~ The thimble-like shape of such sensor element, having
a shoulder at the open end thereof, is illustrated in U.S. 3,978,006 issued
August 31, 1976, and other existing publications, which also describe
various solid electrolyte materials useful in forming such sensor elements.
The preferred composition for forming the solid electrolyte body is a
mixture of zirconium dioxide and stabilizing materials such as calcium
oxide or yttrium oxide.
~ To the interior surface of the electrolyte body, an inner electrode
of conductive catalyst material is applied, such as by the coating of the
surface with a platinum paste with or without a glass frit or other high
temperature-resistance bonding material. This paste coating generally
covers the interior surface of the closed terminal end and exte~ds to the
shoulder of the electrolyte body. m is oombination is then fired at a
temperature of 600-1000C or higher, as is kncwn in the art, for a sufficient
period of time to convert the platinum paste to an electrically conductive
inner elec~rode.
- A glass frit or other bonding agent, when used, while providing
excellent adherence of the catalytic electrode to the interior surface of the
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solid electrolyte body, has an effect of increasing the internal electrical
resistance of the sensor, reducing the positive output voltage of the sensor
when the external surface thereof is exposed to a rich atmosphere and also
causing a negative voltage output when the external surface thereof is
exposed to a lean at~osphere.
As described in our co-pending application, Serial No. 319,639,
the conductive catalyst electrode on the interior surface of the solid
electrolyte body is subjected to a chemical activation treatment to
improve the voltage output and to reduce the internal resistance of the
sensor element. The treatment of the inner conductive catalyst electrode
is by contact of the surface thereof with a solution of an inorganic acid
or an acid salt. Solutions of an inorganic acid, such as hydrochloric
acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid and
chloroplatinic acid, are preferred while acid salts, such as ammonium
chloride, hydroxylamine hydrochloride, ammonium chloroplatinate or the
like, are also usable.
In treating the conductive catalyst electrode with an acidic
or acid slat solution, the electrode may be contacted with the solution
and the same held in contact for a period of time before removing the
solution and rinsing, or the electrode in contact with the solution may be
heated to evaporate solvent from the solution and then heated further to
elevated temperatures in the range of up to 1200C.
In addition to the aforedescribed chemical activation of the
sensor element inner electrode, the outer electrode is subjected to a
current activation treatment.
Both conductive catal~st electrodes, as is known, may comprise
platinum or a platinum family-metal catalyst, such as palladium, rhodium or
mixtures thereof, with the preferred material being platinum.
In the current activation treatment step of the present invention, a
direct current is applied to the sensor element, with the outer conductive
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11192~0
catalyst electrode as an anode, in the presence of a nonoxidizing gas and at
an elevated temperature. m is current ac-tivation is described in detail in
application Serial No. 327,798 of one of the inventors hereof, Ching T. Young,
entitled "Process for Producing a Solid Electrolyte Oxygen Gas Sensing Element,"
filed on even date herewith. As described in said co-pending application,
filed on even date herewith, the outer surface of the solid electrolyte body,
with the outer conductive catalytic electrode thereon, is subjected to a
nonoxidizing atmosphere, and while the outer surface is at a temperature in
excess of 450C, a direct current is applied to the sensing element, with
the outer electrode as an anode, the current density thereof being at least
5 milliamperes per squ æ e centimeter of the planar surface of said outer
conductive catalyst electrode.
The nonoxidizing atm.osphere, to which the outer electrode is
subjected, during the current activation step may be a reducing, neutral
or inert atmosphere, provided that the atmosphere is nonoxidizing. Carbon
monoxide, hydrogen or richexhaust gas mixtures are examples of reducing
atmospheres, while nitrogen is the preferred neutral gas, and argon is an
example of an inert gas. Mixtures of a reducing gas and a neutral or inert
gas may, of course, be used, and a small amount of water vapor may also
be present in the gaseous mixture.
The temperature to which the outer surface is heated prior to
application of the direct current is about 450C and may he as high as about
1100C depending upon the solid electrolyte used and the other process
conditions. A preferred temperature range of 600-900C provides an economical
and efficient temperature range for the current activation.
Application of the direct current is made, to the sensor element,
with the outer conductive catalytic electrode at the elevated temperature and
in the presence of a nonoxidizing gas, with the outer electrode as an anode
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and the inner conductive catalytic electrode as a cathode.
A direct current power source is thus connected to the
conductive catalyst electrodes, with the outer electrode
connected to the positive terminal and the inner electrode
connected to the negative terminal of the power source.
The current charge usa~le in the current
activation step is one which provides a current density of
at least 5 milliamperes per square centimeter af the planar
surface of the outer conductive catalyst electrode. The
term "current density," as used herein, is determined by
dividing the current(in milliamperes) by the planar surface
area of the outer conductive catalyst electrode (cm2) on
the outer surface of the solid electrolyte body, while the
term "planar surface of the outer èlec~rode" is used to
define the surface that would be present if the conductive
catalyst electrode were a smooth coating without porosity.
lhe preferred range of current density is between 20-150
milliamperes per square centimeter of the outer conductive
catalys~ electrode surface. Current densities below 5
milliamperes/cm2 are ineffective to give the beneficial
results and, while much higher current densities can be
used, higher current densities far above the preferred
range can cause fracturing of the element through shock.
The application of the direct current, as above
described, for a period of only about two seconds has been
fQund to provide the desired properties, while a time of
current application of six seconds to about ten minutes is
preferred. The longer times of current application,
however, may require a recovery period for stabilization of
the solid electrolyte. Such a recovery period is effected
by maintaining the outer surface af the sensor element in
the presence of a nonoxidizing gas and at the elevated
temperature for a period of time after the current is
turned off.
The following examples further illustrate the
present invention. In these examples, the testing of
thimbles, as sensor elements, to determine their
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performance in terms of voltage output under rich and lean
conditions, the switching response to gas variation and
their internal resistance, was made by inserting the
thimbles into protective housings with conductive leads
connected to the inner and outer electrodes to form
sensors. The tests were conducted at 350C and a~ 800C
with testing at 800C effected first.
The sensor performance tests were conducted by
inserting the sensors into a cylindrical metal tube and
exposing them to oxidizing and reducing gaseous atmospheres
within the tube through use of a gas burner adjustable to
produce such atmospheres. Sensors placed in the desired
positions in the tube were heated to testing temperature
and the voltage output measured USihg ~ volt meter. The
output was also connected to an oscilloscope to measure the
speed of response of the sensor when the burner flame is
changed from rich to lean and from lean to rich. A routine
test consisted of setting the fLame to rich condition,
measuring the voltage output of the sensor, switching the
flame suddenly to lean condition, triggering the
oscilloscope sweep at the same time to record the rich to
lean switch of the sensor, switching the flame suddenly
back to rich condition, again triggering the oscillo~cope
to record the sensor output change, and finally adjusting
the flame to a lean condition and measuring the sensor
output voltage. The switching time is defined as the time
period required for the output voltage, as recorded on the
oscilloscope, to sweep between 600 and 300 millivolts. When
the sensor output voltage under rich gas condition is less
than 600 millivolts, the switching response time is not
determlnable (n/d) according to the criteria used for this
switching response measurement. Rich voltage output
measurements were then made with different known values of
shunting resistance across the sensor terminals. These
-35 measurements provided data for calculating the internal
resistance of the sensors.
A series of gas sensor electrolyte body thimbles
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92~0
was prepared, for use in the following examples, from
ball-milled zirconia, yttria and alumina, in a ratio of
80%, 14% and 6% by weight respectively, b~ isostatically
pressing the same in the desired thimble shape and firing
at high temperature.
Example I
Three of the series of electrolyte body thimbles
(AEN-1, AEN-2 and AEN-3) had an inner electrode applied to
the inner surface thereof by coating the inner surface with
a platinum suspension containing a glass frit for bonding
purposes. The thimble with its inner electrode was then
heated in an oxidizing atmosphere to burn off the organic
constituents of the suspension and bond the platinum to the
zirconia surface. The external platinum catalyst electrode
was next applied to the outer surface of the thimble by
known thermal vapor deposition. A porous ceramic coating
was applied over the external catalyst layer for
protection. The thimbles were then formed into sensors and
tested as to voltage output, switching response and
internal resistance, as hereinbefore described. The results
of the tests are listed in Table I under the designation
"No Treatment."
The thimbles were then subjected to chemical
treatment by applying to the inner surface thereof an
aqueous solution of one normal hydrochloric acid by filling
the interior portion of the thimbles with the acid. The
sensors were maintained at 50C for a thirty minute period,
and the acid solution was then removed and the interior of
the sensor element washed with distilled water and dried at
100C for at least one hour. These sensor elements were
then again tested as to voltage autput, swltching response
and internal resistance. The results of these tests are
listed in Table I under the heading "After Chemical
Treatment." After this testing was effected, the sensor
elements were subjected to current activation as follows.
The sensor elements, as sensors ~n a protective housing and
with conductive leads, were inserted into a manifold with
--8--
1119250
the outer surface of the sensor element, having the outer
conductive catalyst coating thereon,exposed to a flow of
reducing gas, 0.5% carbon monoxide in nitrogen (with 0.01
mg/cm3 water vapor), at a flow rate of 710 cm3/min. The
elements were preheated to about 700C during a ten~minute
period. The inner conductive catalyst electrode was in
contact with air, and the temperature of the sensor was
taken at the bottom of the inner region of the sensor
element. The sensors were then subjected to a direct
current, as indicated, for a ten-minute period, the direct
current charge applied with the outer electrode as an anode
at a current density of about 167 milliamperes/cm2 of the
outer electrode planar surface, with the ga9 flow continued.
The direct current was then stopped and the sensor element
allowed a recovery pe~iod of ten minutes at said temperature
and with the outer electrode in said gas flow.
These sensor elements were then again tested as
to voltage output, switching response and internal
resistance. The results of these tests are listed in Table
I under the heading "After Chemical Treatment and Current
Activation."
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The tests results shown in Table I indicate the
effect of the chemical treatment upon the voltage output
and internal resistance of the sensor element, and also the
effect o the combination of the chemical treatment and
current activation upon the element, with resultant high
voltage output, low internal resistance and significantly
shortened response time.
Example~
Three other thimbles of the series of electrolyte
- 10 body thimbles (AP7-11, AP7-12 and AP7-13) had inner and
outer electrodes applied thereto as such application was
effected in Example I. These three sensor elements were
then chemically treated by applying to the inner surface
- thereof a 2N aqueous solution (2 gram eguivalent per liter
of solution) of hydrochloric acid. The inner thimble
portion was filled with the acid to cover the inner
electrode, the sensor heated to 50C for 0.5 hr. and, after
pouring out the acid, the inner portion was rinsed twice
with methanol. These three elements were then tested as to
voltage output, switching response and internal resistance.
The results of the tests are listed in Table II under the
designation "Chem. Treated." These three sensor elements
were then sub~ected to current activation by insertion into
a manifold with the outer conductive catalyst coating
exposed to a flow of nitrogen atmosphere (710 cm3/min.)
while the elements were heated to 750C during a ten-minute
period. At a temperature of 750C, and with the outer
electrode subjected to the nitrogen atmosphere, the sensor
elements had applied thereto a direct current, with the
outer electrode as an anode, the current density of which
is listed in Table II for a period of ten minutes. The
direct current was then stopped and the sensor elements
allowed a recovery period of ten minutes, with the outer
conductive catalytic electrode in the flow of nitrogen~ and
;35 at the elevated temperature. These sensor elements were
then again tested. The test results are listed in Table II
;under the heading "After Chemical Treatment and Current
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As illustrated by the test results listed in
Table II, the use of current densities as low as 8 ma/cm
Eor a ten -minute period at 750C are effective in
shortening the response time of the sensor element
following the present process, although the degree of
improvement is not as great as when current densities of 20
or 100 ma/cm2 are used.
Example III
Four other of the series of electrolyte body
thimbles (AP7-10, AP7-21, AP7-22 and AP7-23) had inner and
outer electrodes applied thereto as such application was
effected in Example I. These four sensor elements were then
chemically treated following the procedure described in
Example`II. These four sensor elements were then tested.
The results of the tests are listed in Table III under the
designation "Chem. Treated." These four sens~r elements
were then subjected to current activation according to the
procedure described in Example II, except that the current
density used for each was 100 milliamperes/cm ; the
temperature used for AP7-23 was 600C; and the time of
application of the direct current as well as the recovery
time were varied for the four sensors, these values being
listed in Table III. These four sensor elements were again
tested as to voltage output, switching response and
internal resistance. The results of these tests are listed
in Table III under the heading "After Chemical Treatment
and Current Activation."
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As shown by the test results, where a short time
of current appli.cation is used, for example, 0.1 or 0.03
minute, the need for a recovery period is obviated and the
sensor elements do not appear to require stabilization
after such treatment in order to provide sensor elements of
significantly improved properties.
Example IV
Seven additional thimbles of the series of
electrolyte body thimbles, (AP7-15, AP7-14, AP7-16, AP7-17,
1~ AP7-19, AP7-18 and AP7-20) had inner and outer electrodes
appli.ed as in Example I and were chemically treated
following the procedure described in Example II. These
seven sensor elements were then tested as to voltage
output, switching respQnse and internal resistance. The
results of the tests are listed in Table IV under the
designation "Chem. Treated." The-seven sensor elements were
then current activated according to the procedure described
i.n Example II, except that the temperature for activation,
the current density, and the time of passing of the direct
current were varied for particular of the sensor elements
as indicated in Table IV. The preheating time and recovery
time were both ten minutes in each case. The seven sensor
elements were then again tested. The results of these tests
are l.isted in Table IV under the heading "After Chemical
Treatment and Current Activation."
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The results listed in Table IV illustrate the
effect of various temperatures, current densities and times
of current application on the sensor properties. While the
use of 450C in the test listed did not produce acceptable
sensors for operational purposes, it should be noted that
these sensors had inner electrodes of fluxed platinum
(containing a glass or other bo~ding material) and the use
of such a temperature where no flux is used on the inner
electrode would provide the shortening of the switching
response time desired.
The present process provides a combined chemical
and current treatment of solid electrolyte sensor elements
which results in sensor elements of improved properties of
high voltage output under rich conditions, fast response
time and low internal resistance, all such properties of
which result in an efficient, economical and stable
operation of oxygen gas sensors containing such elements.
-17-
.
