U.S. patent application number 09/828894 was filed with the patent office on 2001-10-04 for nitrogen oxide gas sensor.
Invention is credited to Hasei, Masaharu, Kunimoto, Akira, Yan, Yongtie.
Application Number | 20010025786 09/828894 |
Document ID | / |
Family ID | 16116536 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010025786 |
Kind Code |
A1 |
Hasei, Masaharu ; et
al. |
October 4, 2001 |
Nitrogen oxide gas sensor
Abstract
A nitrogen oxide gas sensor wherein an alloy electrode of
platinum and rhodium or a cermet electrode of platinum, rhodium,
and zirconia or of a rhodium alloy and zirconia is used as the gas
sensing electrode. The electrode of the sensor is suitable for
measuring nitrogen oxide such as NO and NO.sub.2 in an exhaust
gas.
Inventors: |
Hasei, Masaharu;
(Kumagaya-shi, JP) ; Yan, Yongtie; (Kumagaya-shi,
JP) ; Kunimoto, Akira; (Kumagaya-shi, JP) |
Correspondence
Address: |
KUBOVCIK & KUBOVCIK
SUITE 710
900 17TH STREET NW
WASHINGTON
DC
20006
|
Family ID: |
16116536 |
Appl. No.: |
09/828894 |
Filed: |
April 10, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09828894 |
Apr 10, 2001 |
|
|
|
09339108 |
Jun 24, 1999 |
|
|
|
Current U.S.
Class: |
204/424 |
Current CPC
Class: |
G01N 33/0037 20130101;
Y02A 50/20 20180101; G01N 27/4074 20130101; Y02A 50/245
20180101 |
Class at
Publication: |
204/424 |
International
Class: |
G01N 027/407 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 1998 |
JP |
182336/1998 |
Claims
What is claimed is:
1. A nitrogen oxide gas sensor comprising a sensor body; a chamber
formed inside the sensor body; a zirconia solid electrolyte
substrate which is an oxygen ion conductor forming a wall of said
chamber and separating said chamber from an ambient duct; a sensing
electrode provided in the chamber on the solid electrolyte
substrate, said sensing electrode being an alloy electrode
comprising platinum and 0.5-7 wt % rhodium or a cermet electrode
comprising zirconia and an alloy of platinum and 0.5-7 wt %
rhodium; a platinum counter electrode paired to the sensing
electrode, said platinum counter electrode provided in the ambient
duct on the solid electrolyte substrate; and means for controlling
the oxygen concentration at the sensing electrode.
2. A nitrogen oxide gas sensor comprising a sensor body; a chamber
formed inside the sensor body; a zirconia solid electrolyte
substrate which is an oxygen ion conductor forming a wall of said
chamber and separating said chamber from an ambient duct; a gas
sensing electrode provided in the chamber on the solid electrolyte
substrate, wherein the gas sensing electrode of said sensor is an
electrode formed by laminating a rhodium layer having a thickness
of 10-50 .ANG. on a platinum layer or a cermet electrode formed by
dispersing zirconia in each of the rhodium layer and the platinum
layer of the laminated electrode with a rhodium layer of 10-50
.ANG. thickness; a platinum counter electrode paired to the sensing
electrode, said platinum counter electrode provided in the ambient
duct on the solid electrolyte substrate; and a means for
controlling the oxygen concentration at the sensing electrode in a
range of from 0.05 to 21% by volume.
3. A nitrogen oxide gas sensor of claim 2, wherein the rhodium
layer of the sensing electrode is deposited by sputtering
4. A nitrogen oxide gas sensor comprising a sensor body; a chamber
formed in side the sensor body; a zirconia solid electrolyte
substrate which is an oxygen ion conductor forming a wall of said
chamber and separating said chamber from an ambient duct; a gas
sensing electrode provided in the chamber on the solid electrolyte
substrate, wherein the gas sensing electrode of said gas sensor is
made up of an alloy comprising platinum, rhodium and a third metal
element, said third metal element being at least one of Ru, Ir, Pd,
Au, and Ag; a platinum counter electrode paired to the sensing
electrode, said platinum counter electrode provided in the ambient
duct on the solid electrolyte substrate; and a means for
controlling the oxygen concentration at the sensing electrode in a
range of from 0.05 to 21% by volume.
5. A nitrogen oxide gas sensor comprising a sensor body; a chamber
formed inside the sensor body; a zirconia solid electrolyte
substrate which is an oxygen ion conductor forming a wall of said
chamber and separating said chamber from an ambient duct; a gas
sensing electrode provided in the chamber on the solid electrolyte
substrate, wherein the gas sensing electrode of said gas sensor is
made up of a mixed phase rhodium oxide and platinum or an electrode
formed by adding at least one of an oxide of a third metal element
to a mixture and rhodium oxide and platinum, said third metal
element being one of Ru, Ir, Pd, Ag, Ni, and Cr; a platinum counter
electrode paired to the sensing electrode, said platinum counter
electrode provided in the ambient duct on the solid electrolyte
substrate; and a means for controlling the oxygen concentration at
the sensing electrode in a range of from 0.05 to 21% by volume.
6. A nitrogen oxide gas sensor of claim 4, wherein zirconia is
dispersed in said sensing electrode
7. A nitrogen oxide gas sensor of claim 4, wherein in the addition
amount of Rh or the third metal element, the (Rh+third
element)/(Pt+Rh+third element) ratio is from 0.01 to 0.5.
8. A nitrogen oxide gas sensor comprising a sensor body; a chamber
formed inside the sensor body; a zirconia solid electrolyte
substrate which is an oxygen ion conductor forming a wall of said
chamber and separating said chamber from an ambient duct; a gas
sensing electrode provided in the chamber on the solid electrolyte
substrate, wherein the gas sensing electrode of said sensor is an
electrode comprising iridium, an electrode comprising an alloy or a
mixed phase of iridium and rhodium, or a cermet electrode formed by
adding zirconia to iridium or an alloy or a mixed phase of iridium
and rhodium; a platinum counter electrode paired to the sensing
electrode, said platinum counter electrode provided in the ambient
duct on the solid electrolyte substrate; and a means for
controlling the oxygen concentration at the sensing electrode in a
range of from 0.05 to 21% by volume.
9. A nitrogen oxide gas sensor of claim 1, wherein the chamber
consists of first and second chamber in gas communication with each
other, the first chamber is provided with a gas inlet for a gas to
be sensed, the means for controlling the oxygen concentration at
the sensing electrode is located in the first chamber and the
sensing electrode is located in the second chamber.
10. A nitrogen oxide gas sensor of claim 9, wherein the means for
controlling the oxygen concentration at the sensing electrode is an
oxygen pumping electrode.
Description
[0001] This application is a divisional of U.S. patent application.
Ser. No. 09/339,108, filed Jun. 24, 1999, now allowed.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a solid type sensor for a
nitrogen oxide gas, and more specifically to a nitrogen oxide gas
sensor suitable for measuring NO.sub.x discharged from general
combustion system and NO.sub.x in indoor environments, and
particularly for sensing NO.sub.x in the exhaust gas of
automobiles, where the gas temperature may reach quite high
degree.
[0003] A gas sensor of the system of forming a sensing electrode
and a counter electrode thereof on a zirconia solid electrolyte
substrate and sensing the concentration of NO.sub.x by measuring
the potential difference between the electrodes has already been
reported. For example, gas sensors described in Japanese Patent
Laid-Open Publication No. Hei 7-198671 and Japanese Patent
Laid-Open Publication No. Hei 8-43346 each has a construction that
a sensing electrode made up of a metal oxide such as
CdMn.sub.2O.sub.4 or NiCr.sub.2O.sub.4 and a Pt counter electrode
are formed on a zirconia solid electrolyte substrate as an oxygen
ion conductor and it can be said, therefore, that these gas sensors
have a heat resistance capable of being used in a high-temperature
atmosphere.
[0004] On the other hand, as a sensing electrode having a
sufficient heat resistance in a high-temperature atmosphere such as
in an exhaust gas of automobiles, etc., even a noble metal
electrode can be expected to be used. In this point, a Pt electrode
has been already used as an electrode in a .lambda. oxygen sensor
of automobiles and a linear air-fuel oxygen sensor and the high
reliance in the practically used has been already proved. The noble
metal electrode has many merits of a chemical stability thereof,
the easiness of the preparation thereof, capability of expecting
the reduction of the impedance of the electrode, etc. The examples
of the sensor relating to a NO.sub.x gas using a noble metal
sensing electrode on a zirconia solid electrolyte substrate cited
in Japanese Patent Laid-Open Publication No. Hei 8-271476 are as
follows.
[0005] One of them is disclosed in U.S. Pat. No. 4,199,425, which
illustrates a sensor prepared by forming an alumina overcoat layer
impregnated with Rh for imparting a NO.sub.x sensing to a
concentration-cell type oxygen sensor (.lambda. sensor) for
automobiles. However, in this sensor structure, the role of the
overcoat layer impregnated with Rh is a NO.sub.x decomposition
catalyst layer and it is clear that oxygen itself formed by the
decomposition of NOx is sensed by the Pt sensing electrode.
[0006] Another one is shown in Japanese Patent Laid-Open
Publication No. Sho 59-91358, which discloses a sensor prepared by
forming an electrode made up of a noble metal such as Pt, Rh, Pd,
Au, etc., and a sensing electrode prepared by laminating or
applying an N.sub.2O decomposition catalyst such as Co.sub.3O.sub.4
on the above-described electrode on a zirconia solid electrolyte
substrate, and in which the potential difference between the
electrodes is measured. However, in the case of measuring NO.sub.x
in the exhaust gas from automobile engines, the target gases are NO
and NO.sub.2 and thus by the sensor for N.sub.2O, the sum of NO and
NO.sub.2 gases cannot be measured. Furthermore, the potential
difference as sensor outputs to a low-concentration gas is very
small and in the middle concentration range (several 1000 ppm or
lower) of the real exhaust gas, the potential difference is almost
same as zero.
[0007] As described above, although a noble metal sensing electrode
is used in a concentration-cell type NO.sub.x sensor, the role
thereof is simply as a NO.sub.x decomposition catalyst or only
functions as a current collector of collecting electron charges
generated in the decomposition reaction by the catalyst layer.
Furthermore, as mentioned in Japanese Patent Laid-Open Publication
No. Hei 8-271476, it is the present state that in the NO.sub.x
sensor using a conventional noble metal sensing electrode, the
potential difference as sensor outputs is small, the dependence on
the oxygen concentration in a detection gas atmosphere is strong,
and further the sensor can only be operated and sensing with
decomposing NO.sub.x.
[0008] As described above, in the potential difference-type
NO.sub.x sensor using an oxide electrode, a high sensitivity is
obtained but the resistance of the sensing electrode is high and
thus a current collector must be formed in the sensing electrode to
reduce the electrode area.
[0009] On the other hand, in the noble metal electrode which is a
good electric conductor as an electrode material, there is no such
an electrode which measures the NO.sub.x potential difference as it
is. The noble metal electrode can have a little sensitivity only to
N.sub.2O (laughing gas). Furthermore, in the case of noble metal
electrode, it is necessary to control the O.sub.2 concentration
correctly because the potential difference depends on oxygen
partial pressure.
SUMMARY OF THE INVENTION
[0010] In view of these problems, an object of the present
invention is to provide a potential difference-type NO.sub.x sensor
having a low sensor impedance, using a noble metal electrode having
a good electrode conductivity, and having excellent NO.sub.x
sensing characteristics. Furthermore, the object of this invention
is also to provide a potential difference-type NO.sub.x sensor
capable of measuring the NO.sub.x concentration without being
influenced by the oxygen partial pressure of an atmosphere even in
the case of applying it to exhaust gases sensing, etc., from
automobiles.
[0011] The present inventors have found that the above-described
object can be attained by the present invention as set forth
hereinafter.
[0012] That is, the present invention provides (1) a nitrogen oxide
gas sensor of a system of measuring a potential difference between
a sensing electrode formed on a zirconia solid electrolyte
substrate which is an oxygen ion conductor and a platinum counter
electrode or a platinum reference electrode insensitive to
NO.sub.x, which makes a pair with said sensing electrode, on the
above-described solid electrolyte substrate, wherein an alloy
electrode comprising platinum and rhodium or a cermet electrode
comprising platinum, rhodium, and zirconia is used as the gas
sensing electrode of said sensor, and (2) the nitrogen oxide gas
sensor of (1) wherein in the alloy electrode comprising platinum
and rhodium or the cermet electrode comprising platinum, rhodium,
and zirconia, the sensing electrode containing rhodium at least
0.5% by weight to the sum total of platinum and rhodium is
used.
[0013] Also, the present invention provides, as a total NO.sub.x
sensor by combining the alloy electrode comprising platinum and
rhodium or the cermet electrode comprising platinum, rhodium, and
zirconia described above and a sensor structure, (3) a nitrogen
oxide gas sensor of a system of carrying out a sensing by
introducing gas to be sensed into a chamber formed inside the
sensor body each composed of a zirconia solid electrolyte which is
an oxygen ion conductor and having a structure that said chamber is
composed of a 1st chamber having a gas inlet connected to a gas
atmosphere to be sensed or a structure of said 1st chamber and a
2nd chamber connected to the 1st chamber, wherein said sensor
comprising a pair of electrodes formed in the 1st chamber and the
2nd chamber for oxygen pumping-out or oxygen pumping-in, a means
for controlling the oxygen concentration in the 1st chamber or the
2nd chamber, a sensing electrode for NO.sub.x converted into NO or
NO.sub.2 in said 1st chamber, and a platinum counter electrode to
the sensing electrode formed in the chamber where said sensing
electrode placed or a platinum counter electrode formed such that
it connects to a duct of keeping a standard oxygen concentration
putting the zirconia solid electrolyte substrate between said
sensing electrode and said counter electrode, and said sensing
electrode is composed of the alloy electrode comprising platinum
and rhodium or a cermet electrode comprising platinum, rhodium, and
zirconia described above, and (4) the nitrogen oxide gas sensor of
(3) wherein the sensor has a system that the oxygen concentration
in the chamber having formed therein said sensing electrode is
controlled such that the NO.sub.x potential difference is generated
from the mixed potential to oxygen and NO.sub.x in the sensing
electrode. By the nitrogen oxide gas sensor, the oxygen partial
pressure dependence, which becomes a noise of the NO.sub.x sensing,
can be substantially removed.
[0014] Explaining in more detail, platinum and rhodium each is used
as an NO.sub.x catalyst but the alloy of them has never been used
as a potential difference sensing electrode (active to oxygen and
NO.sub.x) itself. Also, the assertion of the present invention is
that the electrode of this invention is used by a different
principle from a conventional concentration cell type. That is, the
mixed potential [the electrode potential (potential difference to
the counter electrode) determined by reactions of NO.sub.x and
O.sub.2 detection on the sensing electrode] determined by
simultaneously taking part in NO.sub.x and oxygen as the
oxidation-reduction reaction of NO.sub.x (NO, NO.sub.2), is used as
an output as the reaction of the NO.sub.x sensing electrode. As the
sensor construction, if a sensing electrode 2 and a counter
electrode 3 (inactive to NO.sub.x) are on a same zirconia solid
electrolyte substrate as shown in FIG. 1 and FIG. 2, there is no
restriction on the arrangement of them. In this case, however,
oxygen must exist in the sensing electrode atmosphere to generate
mixed potential. The counter electrode 3 must be inactive to
NO.sub.x under the using condition and thus is usually formed of
platinum only or formed of platinum added with zirconia for the
control of the electrode microstructure.
[0015] As a matter of course, in the construction shown in FIG. 3,
the reference atmosphere at the counter electrode 3 side may be
fixed to the air.
[0016] Also, when NO.sub.x does not exist in the counter electrode
3 side in the construction shown in FIG. 3, it is clearly in the
category of the present invention that the Pt--Rh alloy electrode,
etc., for example, of this invention having a sensitivity to
NO.sub.x can be used.
[0017] In FIG. 1, FIG. 2, and FIG. 3, 4a and 4b show lead wires
from the electrodes 2 and 3 respectively, and 5 shows an isolation
walls for isolating the counter electrode 3 from a gas to be
sensed.
[0018] Under such a condition, in a conventionally reported oxide
electrode such as NiCr.sub.2O.sub.4, etc., the electric
conductivity of the electrode film itself is low and thus it is
necessary to form a current collector for catching reaction charges
under the electrode. Because the electrode impedance of the oxide
electrode itself is large, for example, when it is used for
automobile, a noise is liable to occur and it is difficult to
ensure the accuracy. Thus, even when it is intended to enlarge the
size of the electrode, because of the low electric conductivity of
the electrode itself, there occurs a problem that the potential
difference can not be measured with good sensitivity without using
a current collector.
[0019] On the other hand, although a noble metal electrode has a
good electric conductivity of electrode, the electrode capable of
sensing NO.sub.x as a mixed potential has not yet been found.
Hitherto, noble metals used for a NO.sub.x sensor of a potential
difference system is from the catalytic property or used as a
simple current collector as described above.
[0020] The present invention is based on the consideration that by
using an alloy film of Pt and Rh for a NO.sub.x sensing electrode,
the oxygen adsorption of Pt and the catalysis of Rh are maintained
on a same electrode, and the NO.sub.x potential difference by the
above-described mixed potential is measured. Accordingly, the
dispersion state of Rh (Rh concentration) and the sensitivity of
NO.sub.x shall have a co-relation and in fact, such a result has
been obtained.
[0021] However, the noble metal electrode is active to oxygen
itself and when the sensing of a concentration cell type is carried
out in, for example, the structure as shown in FIG. 3, the oxygen
concentration fluctuation at the sensing electrode 2 side is
directly sensed, whereby a precise control of the oxygen partial
pressure becomes necessary in the sensing electrode atmosphere. In
the region that an oxygen concentration is almost zero,
practically, the measurement is carried out by only an oxygen
concentration sensor but in this oxygen concentration region, the
output dependence on the oxygen concentration is very strong and
the precise concentration control is substantially impossible.
[0022] On the other hand, in the mixed potential-type sensor which
is the application system of the present invention, the oxygen
concentration dependence is very weak and even when the oxygen
concentration control is substantially very rough, the NO.sub.x
output is scarcely influenced. Thus, even in the circumstance which
is hitherto considered to be used for automobile, the Pt--Rh alloy
electrode of the present system can be practically applied.
[0023] FIG. 7 and FIG. 8 show sensor structures capable of sensing
NO or NO.sub.2 in exhaust gases of automobiles as total NO.sub.x.
In a 1st chamber, NO and NO.sub.2 in the exhaust gas is simplified
into one of NO and NO.sub.2 by an oxygen pumping electrode disposed
in the same chamber and the potential difference measurement is
carried out in the 2nd chamber by the electrode of this invention.
That is, in the case of sensing NO.sub.x as NO.sub.2, oxygen supply
is carried out in the 1st chamber by the pumping electrode and the
oxidation of NO is carried out. Conversely, in the case of
detecting as NO by the reduction of NO.sub.2, the working voltage
of the pump is reversed and oxygen is discharged.
[0024] In any cases, the oxygen concentration in the 1st chamber is
feedback-controlled by the oxygen sensor disposed in the 2nd
chamber connected to the 1st chamber. By incorporating the
above-described mixed potential sensing system into the sensor
structure shown in FIG. 7 or FIG. 8, the oxygen partial pressure
dependence of the conventional noble metal electrode itself is
largely moderated and the electrode can be applied to a sensor
capable of sensing total NO.sub.x as NO.sub.x sensor for
automobile.
[0025] Moreover, the present invention provides a nitrogen oxide
gas sensor of a system of measuring the potential difference
between a sensor electrode formed on a zirconia solid electrolyte
substrate which is an oxygen ion conductor and a Pt reference
electrode without having activity to NO.sub.x on the
above-described solid electrolyte substrate, making a pair with
said sensing electrode, wherein the gas sensing electrode of said
sensor is an electrode formed by laminating a rhodium layer on a
platinum layer or a cermet electrode formed by dispersing zirconia
in the above-described laminated electrode, and the oxygen
concentration of the measuring atmosphere is controlled to a
definite value having an optional width of from 0.05 to 21% by
volume.
[0026] Considering from the circumstance of being used in general
rooms to the circumstance of being used in exhaust gases from
motorcars, the effects of the present invention are as follows.
[0027] In the system of measuring the NO.sub.x concentration by the
potential difference, by using the alloy electrode of Pt--Rh, the
laminated electrode of Pt and Rh, or the cermet electrode of Pt,
Rh, and zirconia each being the electrode in this invention, a very
large sensor output, which is never been obtained by conventional
noble metal electrodes, is obtained. Thereby, the measurement
accuracy of the NO.sub.x concentration is greatly improved.
[0028] By using the alloy electrode of Pt--Rh, the laminated
electrode of Pt and Rh, or the cermet electrode of Pt, Rh, and
zirconia, the electric conductivity of the electrode itself is
improved and it becomes unnecessary to form a current collector on
a sensing electrode.
[0029] In the method of carrying out sintering the electrode with a
zirconia green sheet in a body, the problems of the evaporation of
the electrode material observed in conventional oxide electrode
materials and inferior adhesion do not occur.
[0030] Also, by disposing the electrode of this invention in the
chamber having an oxygen concentration controlled to a certain
extent, the oxygen partial pressure dependence of the electrode
itself is removed and the measurement accuracy in practical sensor
driving is greatly improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a side view showing a fundamental element
construction embodiment (disposed on a same surface) of the
electrode of this invention,
[0032] FIG. 2 is a side view of a fundamental element construction
embodiment (disposed on the front surface and back surface) of the
electrode of this invention,
[0033] FIG. 3 is a side view showing an application example using
the electrode of this invention,
[0034] FIG. 4 is a graph showing the element output characteristics
(NO.sub.2 or NO concentration dependence) of the Pt--Rh (5%)
electrode of this invention, wherein EMF represents an
electromotive force,
[0035] FIG. 5 is a graph showing the Rh composition dependence on
the NO, NO.sub.2 sensitivity of the electrode of this
invention,
[0036] FIG. 6 is a graph showing the controlled oxygen
concentration dependence in the total NO.sub.x sensor
structure,
[0037] FIG. 7 is a cross-sectional view showing an embodiment of
the total NO.sub.x sensor structure to which the electrodes of this
invention are applied,
[0038] FIG. 8 is a cross-sectional view showing other embodiment of
the total NO.sub.x sensor structure to which the electrodes of this
invention are applied,
[0039] FIG. 9 is a cross-sectional view showing a sensor laminated
with Pt and Rh,
[0040] FIG. 10 is a graph showing the sensor output to the Rh film
thickness (A), and
[0041] FIG. 11 is a graph showing the sensor output to an oxygen
concentration.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Then, the present invention is explained in detail.
[0043] In this invention, as a zirconia solid electrolyte
substrate, a known substrate is utilized.
[0044] In this invention, as a formation method of an electrode, a
screen printing method is generally used. For the screen printing
method, a green sheet can be used as a substrate to be printed. As
a matter of course, a sintered substrate can be also used. It is
very useful point that a green sheet can be used in this invention.
This is because an optional form and a complicated laminated layer
structure can be simply formed and also the adhesive property with
a cermet electrode can be more increased than the case of using a
sintered substrate. However, the present invention is not limited
to a green sheet.
[0045] Furthermore, the forming method of the electrode in this
invention is not limited to the forming method by screen printing
and a method of forming a thin film electrode by sputtering, a
method of coating a colloid solution, etc., may be used.
[0046] In the screen printing method of the present invention, as
materials for forming the alloy electrode comprising platinum and
rhodium or the cermet electrode comprising platinum, rhodium, and
zirconia, a paste prepared by kneading the powders of Pt and Rh or
Pt, Rh and ZrO.sub.2 with an organic binder such as PVA, etc., and
a solvent thereof or a dispersing agent thereof are used. As
platinum and rhodium, each powder thereof may be used or a powder
of the alloy of them may be used. When a mixed paste of a platinum
powder and a rhodium powder is fired at a high temperature of at
least 1200.degree. C., they are completely alloyed. This is because
firing at a temperature of at least 1300.degree. C. is necessary
for sintering a zirconia green sheet.
[0047] An addition of zirconia to platinum and rhodium is also
carried out by co-precipitating the material powders in the system
prepared by directly adding a zirconic acid solution to a Pt-acid
solution (same as rhodium solution) at the formation of the
material powders.
[0048] A material simultaneously adding Y.sub.2O.sub.3 to zirconia
to impart an ionic conductivity to it is prepared by the same
manner as above. The addition of zirconia is also effective for
controlling the sintered microstructure of the electrode. The
addition amount of zirconia is controlled according to the sintered
shrinking amount of a zirconia green sheet and the desired
electrode microstructure and is generally from 1 to 20% by weight,
and preferably from 5 to 15% by weight for the electrode
microstructure.
[0049] In the present invention, it is easy to form the
platinum-rhodium electrode on the sensor substrate obtained by
sintering the laminated zirconia green sheet and it is very
effective for improving the NO.sub.x sensitivity. For example, it
is effective to use a zirconia sheet with 8 mols of
Y.sub.2O.sub.3having a high ionic conductivity as the sensor
substrate. In the practical sensor, the addition amount of
Y.sub.2O.sub.3 in the zirconia green sheet is determined from both
the strength characteristics and the long stability of the
substrate. That is, the Y.sub.2O.sub.3 composition which does not
cause a crystal transformation so as to not give a problem to the
long stability and exhibits a high strength is desirable.
[0050] In the present invention, to prepare the laminated electrode
of a platinum layer and a rhodium layer or a cermet electrode
obtained by dispersing zirconia to a platinum-rhodium alloy or the
above-described electrode, in the case of using a screen printing
method, a paste prepared by kneading the material powders for the
electrode with an organic binder such as PVA, etc., and a solvent
or a dispersing agent thereof, etc., are used. To prepare the
laminated electrode described above, first, a Pt film is formed by
printing, sputtering, etc. Thereafter, a Rh film is further formed
by printing, a vapor deposition method, etc. Particularly, in the
case of the vapor-deposited film of Rh, to form a very thin layer,
it is necessary to control the film thickness. Also, in the case of
forming a Rh layer, by oxidizing the Rh layer, the NO.sub.x
sensitivity can be more improved. As a method of forming the oxide
layer of Rh, after printing a Rh layer, the Rh layer may be fired
in the air, or in the case of forming a Rh layer by a
vapor-deposition method, a slight amount of oxygen may be added to
the deposition atmosphere.
[0051] Also, by further adding a third noble metal to the Pt-Rh
electrode, the characteristics can be more improved. For example,
by adding Ru, Ir, Pd, Au, or Ag to the Pt-Rh electrode, the
gas-responsibility of the electrode can be greatly improved. This
is considered to be caused by that the electrode microstructure
becomes fine without reducing the NO.sub.x activity of the Pt--Rh
electrode. This shows that the electrode impedance is substantially
further reduced.
[0052] On the other hand, it has been found that iridium, an alloy
of iridium and rhodium, or a mixed phase electrode also has a large
NO.sub.x activity. Particularly, in an iridium electrode, it
becomes a very fine electrode microstructure and the high
gas-response speed is obtained. These electrodes are prepared by
the same manner of preparing the above-described Pt-Rh electrodes
For example, a paste using an iridium powder is printed on a green
sheet and may be fired. Also, a paste is prepared from the alloy
powder of iridium and rhodium or a mixture of an iridium powder and
a rhodium powder and the electrode is similarly prepared using the
paste. However, in this case, it sometimes happens that a part of
the Rh phase is separated and precipitated to form a mixed
phase.
[0053] Apart from this, as the sensing electrode, a mixed phase of
platinum and rhodium oxide or an electrode prepared by mixing at
least one of the oxides of Ru, Ir, Pd, Ag, Ni, and Cr with the
above-described mixed phase may be used.
[0054] The oxygen concentration in the measuring atmosphere may be
within the range of from 0.5 to 21% by volume. Accordingly, the
oxygen concentration is controlled to the range of from 0.5 to 21%
by volume.
[0055] Then, the present invention is explained by showing the
detailed examples.
EXAMPLE 1
[0056] Fundamental preparation method and the characteristics in
the present invention:
[0057] A sensor sample of the structure shown in FIG. 1 was
prepared using a green sheet 1 of zirconia added with 8 mols of
Y.sub.2O.sub.3 as an oxygen ion conductor. The green sheet was
prepared by a doctor blade method and had a thickness of 0.3 mm.
The green sheet was cut into a sample size of 4 mm.times.6 mm. As
the material for the sensing electrode 2, a paste was prepared by
kneading the alloy powder of Pt and Rh with definite amounts of an
organic binder and an organic solvent. The addition amount of Rh
was 5% by weight to the sum total of Pt and Rh. To control the
porosity of the electrode, zirconia was further added to the paste.
As a reference electrode 3, Pt paste was printed on the surface of
the above-described zirconia sheet 1 such that it made a pair with
the sensing electrode 2 and zirconia was dispersed in the reference
electrode 3 to control the electrode microstructure, as the case of
the sensing electrode 2.
[0058] The sample of a green sheet thus prepared was fired at
1400.degree. C. and after connecting Pt lead wires 4a, 4b to the
electrodes 2, 3 respectively, the evaluation of NO and NO.sub.2 gas
sensitivity was carried out. For the gas sensitivity evaluation, a
quartz tube was placed in an electric furnace, the sample was
inserted in the quartz tube, and while flowing gas to be measured,
the potential difference between the sensing electrode 2 and the
reference electrode 3 was measured. The gas to be measured was an
N.sub.2-base gas added with 4% O.sub.2 and 50 ppm of NO or NO.sub.2
and measurement was carried out at a total flow rate of 5
liters/minute. As the measurement temperature, the temperature of
the electric furnace was controlled by a thermocouple placed
adjacent to the sensor sample, and the measurement was carried out
at an atmospheric temperature of 600.degree. C. FIG. 4 shows the
result of the NO.sub.x concentration dependence of the sensor
output to NO.sub.2 and NO. From the result, it can be seen that the
sensitivity to NO.sub.2 shows the output of same as or higher than
that conventionally reported for NiCr.sub.2O.sub.4 sensing
electrode. Also, it can be seen that the electrode has a
sensitivity to NO.
EXAMPLE 2
[0059] Sensor samples each having a changed Rh composition ratio
were prepared by the same manner as in Example 1. In this case,
however, to control the composition ratio of Pt and Rh, mixed
powder of Pt powder and Rh powder was used. The composition ratios
of Rh were 0.1%, 0.5%, 1.0%, 3.0%, 5.0%, 7.0%, 50%, and 100% (by
weight) to the sum total of Pt and Rh. For the sensitivity
measurement, the same apparatus as used in Example 1 was used and
the sensitivity to 50 ppm of NO or 50 ppm of NO.sub.2 was evaluated
at an atmospheric temperature of 600.degree. C. and the total gas
flow rate of 5 liters/minute. The results are shown in Table 1 and
FIG. 5. From the results, it can be seen that the sensor samples
having the Rh composition ratio of 0.5% by weight or higher have a
large sensitivity to NO.sub.2.
[0060] On the other hand, it can be seen that the sensor samples
having the Rh composition ratio of from 0.5% to 50% by weight have
a sensitivity to NO.
1TABLE 1 Rh Composition ratio NO Sensitivity NO.sub.2 Sensitivity
0.1 wt. % -0.1 mV 0.8 mV 0.5 wt. % -4.3 mV 30.3 mV 1.0 wt. % -13.6
mV 50.2 mV 3.0 wt. % -17.9 mV 68.8 mV 5.0 wt. % -15.4 mV 58.5 mV
7.0 wt. % -6.0 mV 51.5 mV 50 wt. % -3.0 mV 29.4 mV 100 wt. % -0.1
mV 24.7 mV
EXAMPLE 3
[0061] A sample was prepared almost same as in Example 1 but in
this example, the sensor structures shown in FIG. 7 and FIG. 8
respectively were constructed.
[0062] Between a substrate 6 of a zirconia solid electrolyte for an
oxygen pump and a substrate 7 of a zirconia solid electrolyte for
an NO.sub.x sensor and an oxygen sensor, facing each other, is
interposed a spacer 19 having a 1st inlet 12 for a gas to be
measured and 2nd inlet 13 separated from and opposite to the 1st
inlet 12, and a 1st chamber 14 and a 2nd chamber 15 are formed. The
substrate 6 has oxygen pump electrodes 9a and 9b on both the
surfaces thereof at the 1st chamber 14 side, and the substrate 7
has an NO.sub.x sensing electrode 10a and an NO.sub.x counter
electrode 10b thereof, and an oxygen sensing electrode 11a and an
oxygen counter electrode 11b, on both the surfaces thereof at the
2nd chamber side. On the other hand, the oxygen pump electrode 9b
is exposed in the 1st chamber 14, and the NO.sub.x sensing
electrode 10a and the oxygen sensing electrode 11a are exposed in
the 2nd chamber 15.
[0063] In addition, in the embodiment of FIG. 8, both the NO.sub.x
sensing electrode 10a and the NO.sub.x counter electrode 10b are
exposed in the 2nd chamber 15 but other construction of the
embodiment of FIG. 8 than the above-described point is same as the
construction of FIG. 7.
[0064] An oxygen-introducing duct partition wall 8b for the
NO.sub.x sensor is disposed over the substrate 6 via a spacer 20
facing the substrate to form an oxygen introducing duct 17 for the
oxygen pump, and also a standard atmospheric duct partition wall 8a
is disposed under the substrate 7 via spacer 21 facing the
substrate to form a standard atmospheric duct 16 for the NO.sub.x
sensor and the oxygen sensor. Both ducts 16 and 17 are opened to a
standard atmosphere (the air) 18.
[0065] The potential difference V.sub.1 between the NO.sub.x
sensing electrode 10a and the NO.sub.x counter electrode 10b and
the potential differences V.sub.2 between the oxygen sensing
electrodes 11a and the counter electrode 11b are measured.
[0066] In the sensor structures of the examples of this invention
shown in FIG. 7 and FIG. 8, the oxygen concentration in an exhaust
gas introduced in the chamber 14 is controlled by the pump
electrodes 9a and 9b and NO.sub.x is simplified into NO or
NO.sub.2. The NO.sub.x simplified into NO or NO.sub.2 is sensed as
a potential difference by the Pt--Rh (5%) electrodes 10a and Pt
electrode 10b in the connected chamber 15. In this case, the oxygen
concentration in the chamber 15 is measured by the oxygen sensing
electrode 11a and oxygen counter electrode 11b and adjusted to a
definite concentration range by the pump electrodes 9a and 9b. By
the electrodes 10a and 10b of this invention formed in the chamber
15, NO or NO.sub.2 is sensed by a single output V.sub.1 of NO or
NO.sub.2. In the example, the total NO.sub.x output characteristics
of a mixed gas of (NO: 25 ppm) and (NO.sub.2: 25 ppm) were
evaluated in both cases of the NO.sub.2 sensing system and the NO
sensing system when the oxygen concentration in the chamber 15 is
adjusted to the concentration range of from 4% to 50%.
[0067] As is clear from the results shown in FIG. 6, it can be seen
that by applying the electrode of this invention to each of the
sensor structures shown in the example, NO.sub.x (NO and NO.sub.2)
in the exhaust gas is sensed as a total NO.sub.x concentration as
well as the strong oxygen concentration dependence of the Pt--Rh
sensing electrode itself is removed and a stable NO.sub.x sensing
is carried out. That is, in the concentration of oxygen of near 4%
which the oxygen concentration dependence is strongest in the
measuring range by the NO.sub.2 sensing system, even by a rough
control of the oxygen concentration of .+-.1%, the accuracy of
.+-.2.5 ppm can be ensured at a low concentration range sensitivity
of NO.sub.x: 50 ppm.
[0068] On the other hand, it can be seen that in the NO sensing
system, the output is almost saturated in the range of high oxygen
concentration and when the oxygen concentration is substantially
10% or higher, there is no problem.
EXAMPLE 4
[0069] As an oxygen ion conductor, a sensor sample of the structure
shown in FIG. 9 was prepared using a green sheet of zirconia added
with 8 mols of Y.sub.2O.sub.3. The green sheet was prepared by a
doctor blade method and had a thickness of 0.3 mm. The zirconia
green sheet 25 was cut into a sample size of 4 mm.times.6 mm. Pt
paste was prepared by adding a definite amount of an organic binder
(for example, PVA) to Pt as a sensing electrode material followed
by kneading. Furthermore, Rh paste was prepared by adding a
definite amount of an organic binder (for example, PVA) to Rh as a
sensing electrode material followed by kneading. First, the Pt
paste 26 was formed on the green sheet by screen printing and after
drying in an oven, the Rh paste 27 was formed thereon by printing.
As a reference electrode 28, the Pt paste was printed on the back
surface of the above-described zirconia green sheet 25 so as to
make a pair with the sensing electrodes 26 and 27. The sample of a
green sheet thus prepared was fired at 1400.degree. C. and after
connecting Pt lead wires to the electrodes, the evaluation of the
NO and NO.sub.2 gas sensitivity was carried out. The Rh film
thickness of the sample obtained was about 3 .mu.m and the Rh film
was porous. In the evaluation of the gas sensitivity, a quartz tube
was placed in an electric furnace, the sample was inserted in the
quartz tube and while flowing gas to be measured, the potential
difference between the sensing electrodes 26, 27 and the reference
electrode 28 was measured. The gas to be measured was an N.sub.2
base gas added with 4% O.sub.2 and 300 ppm of NO or NO.sub.2 and
the measurement was carried out at a total flow rate of 5
liters/minute. As the measurement temperature, the temperature of
the electric furnace was controlled by a thermocouple placed
adjacent to the sensor sample, and the measurement was carried out
at an atmospheric temperature of 600.degree. C. In Table 2, the
results of the sensor outputs to NO.sub.2 and NO are shown together
with the results obtained by using a conventional oxide electrode
for comparison. As clear from the results, the sensitivity to
NO.sub.2 shows the output of same as or higher than that
conventionally reported for the NiCr.sub.2O.sub.4 sensing
electrode. Also, it can be seen that the electrode of this
invention also has a sensitivity to NO.
[0070] In addition, a sensing electrode prepared by adding zirconia
(10% by weight) to each of the pastes described above showed almost
the same results as those in Table 2.
2TABLE 2 Sensing electrode NO 300 ppm NO.sub.2 300 ppm
NiCr.sub.2O.sub.4 -25 mV 97 mV Pt/Rh (Laminated) -20 mV 91 mV
EXAMPLE 5
[0071] A sensor sample was prepared by the same method as in
Example 4. In this case, however, as the NO.sub.x sensing
electrode, a Pt electrode same as the reference electrode was
formed. On the Pt sensing electrode, Rh was deposited by a
sputtering method. The relations between the Rh film thickness
over-coated, obtained from the relation between the sputtering time
and the Rh film thickness, and the 50 ppm NO sensitivity or the 50
ppm NO.sub.2 sensitivity are shown in FIG. 10. As is seen from the
results, the sensitivity almost tends to be increased from the
over-coated film thickness of about 10 angstroms and saturated at
the vicinity of 40 angstroms. As described above, it can be seen
that a very large sensitivity is obtained by forming the very thin
Rh layer.
EXAMPLE 6
[0072] Samples were prepared by almost same manner as in Example 4
but in this case, as the sensing electrode materials, Pt--Rh--X was
used and as X, each of noble metals, Ru, Ir, Pd, Au, and Ag was
added in an amount of from 1 to 10% by weight. About these samples,
the NO.sub.x sensitivity was measured as in Example 4.
[0073] These results are shown in Table 3. From the results, it can
be seen that in each of the samples, the NO.sub.x sensitivity is
sufficiently large and the electrode impedance is reduced.
3TABLE 3 NO.sub.2 Electrode Sensing Electrode NO Sensitivity
Sensitivity Impedance Pt--Rh(3 wt %) -12 mV 102 mV 232 K.OMEGA.
Pt--Rh(3 wt %)--Ru(1 wt %) -26 mV 111 mV 44 K.OMEGA. Pt--Rh(3 wt
%)--Ru(5 wt %) -19 mV 100 mV 37 K.OMEGA. Pt--Rh(3 wt %)--Ru(10 wt
%) -13 mV 81 mV 22 K.OMEGA. Pt--Rh(3 wt %)--Ir(1 wt %) -7 mV 48 mV
49 K.OMEGA. Pt--Rh(3 wt %)--Ir(5 wt %) -14 mV 78 mV 73 K.OMEGA.
Pt--Rh(3 wt %)--Ir(10 wt %) -21 mV 97 mV 102 K.OMEGA. Pt--Rh(3 wt
%)--Pd(1 wt %) -11 mV 69 mV 87 K.OMEGA. Pt--Rh(3 wt %)--Pd(5 wt %)
-9 mV 54 mV 40 K.OMEGA. Pt--Rh(3 wt %)--Pd(10 wt %) -5 mV 36 mV 34
K.OMEGA. Pt--Rh(3 wt %)--Au(1 wt %) -15 mV 81 mV 85 K.OMEGA.
Pt--Rh(3 wt %)--Au(5 wt %) -14 mV 81 mV 71 K.OMEGA. Pt--Rh(3 wt
%)--Au(10 wt %) -11 mV 72 mV 66 K.OMEGA. Pt--Rh(3 wt %)--Ag(1 wt %)
-19 mV 91 mV 60 K.OMEGA. Pt--Rh(3 wt %)--Ag(5 wt %) -6 mV 57 mV 288
K.OMEGA. Pt--Rh(3 wt %)--Ag(10 wt %) -3 mV 36 mV 150 K.OMEGA.
Pt--Rh(10 wt %)--Ru(5 wt %) -16 mV 72 mV 96 K.OMEGA. Pt--Rh(20 wt
%)--Ru(5 wt %) -10 mV 47 mV 88 K.OMEGA. Pt--Rh(45 wt %)--Ru(5 wt %)
-6 mV 34 mV 133 K.OMEGA.
EXAMPLE 7
[0074] After preparing sensor samples as in Example 4, they were
subjected to an oxidation treatment at 850.degree. C. In this case,
however, as the detection electrode, each of Pt--Rh and Pt--Rh--X
(wherein X is Ru, Pd, Ir, Ni, or Cr) was used. The results are
shown in Table 4. In the sensing electrodes of the samples
obtained, the Rh oxide and the oxide of the third addition element
were confirmed by an X-ray diffraction or the surface analysis of
XPS. In each case, it can be seen that by applying the oxidation
treatment, the sensitivity is largely improved as compared with the
electrode before applying the oxidation treatment.
4TABLE 4 Before oxidation After oxidation Sensing Electrode
treatment treatment Material NO.sub.2 Sensitivity NO.sub.2
Sensitivity Pt--Rh(5 wt %) 59 mV 102 mV Pt--Rh(5 wt %)--Ru(5 wt %)
56 mV 100 mV Pt--Rh(5 wt %)--Ir(5 wt %) 46 mV 75 mV Pt--Rh(5 wt
%)--Pd(5 wt %) 34 mV 54 mV Pt--Rh(5 wt %)--Ag(5 wt %) 52 mV 88 mV
Pt--Rh(5 wt %)--NiO(5 wt %) 61 mV 85 mV Pt--Rh(5 wt
%)--Cr.sub.2O.sub.3(5 wt %) 65 mV 93 mV
Example 8
[0075] Sensor samples were prepared as in Example 1. As the sensing
electrodes in the example, an Ir electrode and Ir--Rh alloy
electrodes were used. The results of the sensitivity measurement of
them are shown in Table 5. From the results, it can be seen that
sufficiently large NO.sub.x sensitivity is obtained and also, a
good gas-responsibility is obtained.
5 TABLE 5 Sensing Electrode 300 ppm NO 300 ppm NO.sub.2 Material
Sensitivity Sensitivity Ir -4 mV 110 mV Ir--Rh(3 wt %) -10 mV 119
mV Ir--Rh(7 wt %) -9 mV 115 mV Ir--Rh(10 wt %) -6 mV 112 mV
[0076] In addition, in FIG. 11, the oxygen concentration dependence
of an NO.sub.x sensor using a Pt--Rh (3 wt. %) sensing electrode is
shown. From FIG. 11, it can be seen that the oxygen concentration
should be controlled to a definite value having the range of from
0.05 to 21% by volume.
[0077] If the oxygen concentration is lower than 0.05% by volume,
the gas-response speed is extremely delayed, which is undesirable.
Also, in the chamber structure, the oxygen concentration necessary
for removing by oxidation HC and CO in an exhaust gas is preferably
from 0.5 to 21% by volume and more preferably from 0.5 to 5% by
volume considering the sensor output.
* * * * *