U.S. patent application number 09/212491 was filed with the patent office on 2002-01-17 for nox concentration-measuring apparatus.
Invention is credited to KATO, NOBUHIDE, NAKAGAKI, KUNIHIKO, NISHIKAWA, SATOSHI.
Application Number | 20020005353 09/212491 |
Document ID | / |
Family ID | 18432891 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005353 |
Kind Code |
A1 |
KATO, NOBUHIDE ; et
al. |
January 17, 2002 |
NOX CONCENTRATION-MEASURING APPARATUS
Abstract
Disclosed is a NOx sensor for measuring a NOx concentration
comprising a main pumping cell and a detecting electrode, the main
pumping cell including an electrode (an inner pumping electrode and
an outer pumping electrode) having no decomposing/reducing ability
for NOx or a low decomposing/reducing ability for NOx, to be used
so that an oxygen concentration in a measurement gas is controlled
to have a predetermined value at which NO is not substantially
decomposable, and the detecting electrode having a certain
decomposing/reducing ability for NOx or a high decomposing/reducing
ability for NOx, to be used so that NOx is decomposed to measure
the NOx concentration by measuring an amount of oxygen produced
during this process, wherein a cermet electrode composed of a
Pt--Rh alloy and a ceramic component is used as the detecting
electrode. Accordingly, it is possible to suppress the oxidation of
Rh and the reconversion into the metal contained in the detecting
electrode, stabilize the impedance, and stabilize the sensitivity
to NOx.
Inventors: |
KATO, NOBUHIDE; (AMA-GUN,
JP) ; NAKAGAKI, KUNIHIKO; (NAGOYA-CITY, JP) ;
NISHIKAWA, SATOSHI; (CHITA-CITY, JP) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Family ID: |
18432891 |
Appl. No.: |
09/212491 |
Filed: |
December 17, 1998 |
Current U.S.
Class: |
204/426 ;
205/781 |
Current CPC
Class: |
G01N 27/417 20130101;
G01N 33/0037 20130101; G01N 27/4075 20130101; Y02A 50/20 20180101;
G01N 27/4074 20130101; G01N 27/419 20130101; Y02A 50/245
20180101 |
Class at
Publication: |
204/426 ;
205/781 |
International
Class: |
G01N 027/407 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 1997 |
JP |
9-353738 |
Claims
What is claimed is:
1. A NOx concentration-measuring apparatus for measuring a NOx
concentration, comprising an oxygen pump and a NOx-decomposing
electrode; said oxygen pump including an electrode having no
decomposing/reducing ability for NOx or a low decomposing/reducing
ability for NOx, to be used so that an oxygen concentration in a
measurement gas is controlled to have a predetermined value at
which NO is not substantially decomposable; and said
NOx-decomposing electrode having a certain decomposing/reducing
ability for NOx or a high decomposing/reducing ability for NOx, to
be used so that NOx is decomposed to measure said NOx concentration
by measuring an amount of oxygen produced during this process,
wherein: a cermet electrode composed of a Pt--Rh alloy and a
ceramic component is used as said NOx-decomposing electrode.
2. The NOx concentration-measuring apparatus according to claim 1,
wherein a weight ratio of Rh in said Pt--Rh alloy contained in said
NOx-decomposing electrode is more than 0% by weight and not more
than 90% by weight.
3. The NOx concentration-measuring apparatus according to claim 2,
wherein a ratio of Pt to Rh in said NOx-decomposing electrode is
Pt:Rh=10:90 to 99:1 as represented by a weight ratio.
4. The NOx concentration-measuring apparatus according to claim 2,
wherein said weight ratio of Pt in said Pt--Rh alloy contained in
said NOx-decomposing electrode is not less than 25% by weight, and
Pt is not contained in an amount of 100% by weight.
5. The NOx concentration-measuring apparatus according to claim 4,
wherein a ratio of Pt to Rh in said NOx-decomposing electrode is
Pt:Rh=25:75 to 75:25 as represented by a weight ratio.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a NOx
concentration-measuring apparatus for measuring NOx contained in,
for example, atmospheric air and exhaust gas discharged from
vehicles or automobiles.
[0003] 2. Description of the Related Art
[0004] Conventionally, those known as the method for measuring NOx
in a measurement gas such as combustion gas include a technique in
which the NOx-reducing ability of Rh is utilized to use a sensor
comprising a Pt electrode and an Rh electrode formed on an oxygen
ion-conductive solid electrolyte such as zirconia so that an
electromotive force generated between the both electrodes is
measured.
[0005] However, the sensor as described above suffers the following
problem. That is, the electromotive force is greatly changed
depending on the change in concentration of oxygen contained in the
combustion gas as the measurement gas. Moreover, the change in
electromotive force is small with respect to the change in
concentration of NOx. For this reason, the conventional sensor
tends to suffer influence of noise.
[0006] Further, in order to bring out the NOx-reducing ability, it
is indispensable to use a reducing gas such as CO. For this reason,
the amount of produced CO is generally smaller than the amount of
produced NOx under a lean fuel combustion condition in which a
large amount of NOx is produced. Therefore, the conventional sensor
has a drawback in that it is impossible to perform measurement for
a combustion gas produced under such a combustion condition.
[0007] In order to solve the problems as described above, for
example, Japanese Laid-Open Patent Publication No. 8-271476
discloses a NOx sensor comprising pumping electrodes having
different NOx-decomposing abilities arranged in a first internal
space which communicates with a measurement gas-existing space and
in a second internal space which communicates with the first
internal space, and a method for measuring the NOx concentration in
which the O.sub.2 concentration is adjusted by using a first
pumping cell arranged in the first internal space, and NO is
decomposed by using a decomposing pumping cell arranged in the
second internal space so that the NOx concentration is measured on
the basis of a pumping current flowing through the decomposing
pump.
[0008] Further, Japanese Laid-Open Patent Publication No. 9-113484
discloses a sensor element comprising an auxiliary pumping
electrode arranged in a second internal space so that the oxygen
concentration in the second internal space is controlled to be
constant even when the oxygen concentration is suddenly
changed.
[0009] A cermet electrode composed of Rh/ZrO.sub.2 is used for the
NOx-decomposing electrode as described above. When the cermet
electrode composed of Rh/ZrO.sub.2 is used for the NOx-decomposing
electrode, a phenomenon has been observed, in which the sensitivity
is lowered in accordance with the increase in operating time. This
phenomenon is caused by the increase in impedance of the
decomposing pumping cell. When the NOx sensor element, in which the
impedance has been increased, is observed, it has been found that
the contact area is decreased between the NOx-decomposing electrode
and the ZrO.sub.2 substrate. In other words, it is postulated that
the increase in impedance is caused by the decrease in contact area
between the NOx-decomposing electrode and the ZrO.sub.2
substrate.
[0010] It is postulated that the decrease in contact area between
the NOx-decomposing electrode and the ZrO.sub.2 substrate is caused
by any change in volume, which is brought about by the oxidation
(Rh.sub.2O.sub.3) of the metal Rh contained in the NOx-decomposing
electrode and the reconversion of the oxidized product into the
metal.
SUMMARY OF THE INVENTION
[0011] The present invention has been made taking the foregoing
problems into consideration, an object of which is to provide a NOx
concentration-measuring apparatus which makes it possible to
suppress the oxidation of Rh and the reconversion into the metal
contained in a NOx-decomposing electrode, stabilize the impedance,
and stabilize the measurement sensitivity.
[0012] The present invention lies in a NOx concentration-measuring
apparatus for measuring a NOx concentration, comprising an oxygen
pump and a NOx-decomposing electrode, the oxygen pump including an
electrode having no decomposing/reducing ability for NOx or a low
decomposing/reducing ability for NOx, to be used so that an oxygen
concentration in a measurement gas is controlled to have a
predetermined value at which NO is not substantially decomposable,
and the NOx-decomposing electrode having a certain
decomposing/reducing ability for NOx or a high decomposing/reducing
ability for NOx, to be used so that NOx is decomposed to measure
the NOx concentration by measuring an amount of oxygen produced
during this process, wherein a cermet electrode composed of a
Pt--Rh alloy and a ceramic component is used as the NOx-decomposing
electrode.
[0013] The use of the cermet electrode composed of the Pt--Rh alloy
and the ceramic component, as the NOx-decomposing electrode
suppresses the oxidation of Rh and the reconversion of the oxidized
product into the metal contained in the NOx-decomposing electrode.
Even when the operating time is increased, it is possible to avoid
the occurrence of the increase in impedance which would be
otherwise caused by the decrease in contact area between the
NOx-decomposing electrode and the substrate. That is, when the NOx
concentration-measuring apparatus according to the present
invention is used, then the impedance is stabilized, and it is
possible to stabilize the measurement sensitivity as well.
[0014] It is appropriate that a weight ratio of Rh in the Pt--Rh
alloy contained in the NOx-decomposing electrode is more than 0% by
weight and not more than 90% by weight.
[0015] Specifically, a ratio of Pt to Rh in the NOx-decomposing
electrode is preferably Pt:Rh=10:90 to 99:1 as represented by a
weight ratio. It is especially desirable that the weight ratio of
Pt in the Pt--Rh alloy contained in the NOx-decomposing electrode
is not less than 25% by weight, and Pt is not contained in an
amount of 100% by weight. It is more preferable that the ratio of
Pt to Rh in the NOx-decomposing electrode is Pt:Rh=25:75 to 75:25
as represented by a weight ratio.
[0016] The above and other objects, features, and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying
drawings in which a preferred embodiment of the present invention
is shown by way of illustrative example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows an arrangement of a NOx sensor according to an
embodiment of the present invention;
[0018] FIG. 2 shows characteristics illustrating results of a first
illustrative experiment (illustrative experiment to measure the way
of change in mass of alloys composed of Rh and Pt, in accordance
with the increase in heat, by changing the weight ratio of Rh and
Pt);
[0019] FIG. 3 shows limiting current characteristics obtained upon
application of heating at 800.degree. C. in the atmospheric air in
a second illustrative experiment;
[0020] FIG. 4 shows limiting current characteristics obtained upon
application of heating at 700.degree. C. in the atmospheric air in
the second illustrative experiment;
[0021] FIG. 5A shows the change in impedance with respect to the
durable time in a third illustrative experiment;
[0022] FIG. 5B shows the change in NOx sensitivity with respect to
the durable time in the third illustrative experiment; and
[0023] FIG. 6 shows an arrangement of a NOx sensor according to a
modified embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Explanation will be made below with reference to FIGS. 1 to
6 for illustrative embodiments of the NOx concentration-measuring
apparatus according to the present invention (hereinafter simply
referred to as "NOx sensor according to the embodiment").
[0025] At first, as shown in FIG. 1, a NOx sensor 10 according to
the embodiment of the present invention comprises, for example, six
stacked solid electrolyte layers 12a to 12f composed of ceramics
based on the use of oxygen ion-conductive solid electrolytes such
as ZrO.sub.2. First and second layers from the bottom are
designated as first and second substrate layers 12a, 12b
respectively. Third and fifth layers from the bottom are designated
as first and second spacer layers 12c, 12e respectively. Fourth and
sixth layers from the bottom are designated as first and second
solid electrolyte layers 12d, 12f respectively.
[0026] Specifically, the first spacer layer 12c is stacked on the
second substrate layer 12b. The first solid electrolyte layer 12d,
the second spacer layer 12e, and the second solid electrolyte layer
12f are successively stacked on the first spacer layer 12c.
[0027] A space (reference gas-introducing space) 14, into which a
reference gas such as atmospheric air to be used as a reference for
measuring oxides is introduced, is formed between the second
substrate layer 12b and the first solid electrolyte layer 12d, the
space 14 being comparted by a lower surface of the first solid
electrolyte layer 12d, an upper surface of the second substrate
layer 12b, and side surfaces of the first spacer layer 12c.
[0028] The second spacer layer 12e is interposed between the first
and second solid electrolyte layers 12d, 12f. First and second
diffusion rate-determining sections 16, 18 are also interposed
between the first and second solid electrolyte layers 12d, 12f.
[0029] A first chamber 20 for adjusting the partial pressure of
oxygen in a measurement gas is formed and comparted by a lower
surface of the second solid electrolyte layer 12f, side surfaces of
the first and second diffusion rate-determining sections 16, 18,
and an upper surface of the first solid electrolyte layer 12d. A
second chamber 22 for finely adjusting the partial pressure of
oxygen in the measurement gas and measuring oxides, for example,
nitrogen oxides (NOx) in the measurement gas is formed and
comparted by a lower surface of the second solid electrolyte layer
12f, a side surface of the second diffusion rate-determining
section 18, a side surface of the second spacer layer 12e, and an
upper surface of the first solid electrolyte layer 12d.
[0030] The external space communicates with the first chamber 20
via the first diffusion rate-determining section 16, and the first
chamber 20 communicates with the second chamber 22 via the second
diffusion rate-determining section 18.
[0031] The first and second diffusion rate-determining sections 16,
18 give predetermined diffusion resistances to the measurement gas
to be introduced into the first and second chambers 20, 22
respectively. Each of the first and second diffusion
rate-determining sections 16, 18 can be formed as a passage
composed of, for example, a porous material, or a small hole having
a predetermined cross-sectional area so that the measurement gas
may be introduced.
[0032] Especially, a porous material composed of, for example,
ZrO.sub.2 is charged and arranged in the second diffusion
rate-determining section 18. The diffusion resistance of the second
diffusion rate-determining section 18 is made larger than the
diffusion resistance of the first diffusion rate-determining
section 16. The diffusion resistance of the second diffusion
rate-determining section 18 is preferably larger than that of the
first diffusion rate-determining section 16. However, no problem
occurs even when the former is smaller than the latter.
[0033] The atmosphere in the first chamber 20 is introduced into
the second chamber 22 under the predetermined diffusion resistance
via the second diffusion rate-determining section 18.
[0034] An inner pumping electrode 24 having a substantially
rectangular planar configuration and composed of a porous cermet
electrode (for example, a cermet electrode composed of
Pt.multidot.ZrO.sub.2 containing 1% by weight of Au) is formed on
the entire lower surface portion for forming the first chamber 20,
of the lower surface of the second solid electrolyte layer 12f. An
outer pumping electrode 26 is formed on a portion corresponding to
the inner pumping electrode 24, of the upper surface of the second
solid electrolyte layer 12f. An electrochemical pumping cell, i.e.,
a main pumping cell 28 is constructed by the inner pumping
electrode 24, the outer pumping electrode 26, and the second solid
electrolyte layer 12f interposed between the both electrodes 24,
26.
[0035] A desired control voltage (pumping voltage) Vp1 is applied
between the inner pumping electrode 24 and the outer pumping
electrode 26 of the main pumping cell 28 by the aid of an external
variable power source 30 to allow a pumping current Ip1 to flow in
a positive or negative direction between the outer pumping
electrode 26 and the inner pumping electrode 24. Thus, the oxygen
in the atmosphere in the first chamber 20 can be pumped out to the
external space, or the oxygen in the external space can be pumped
into the first chamber 20.
[0036] A reference electrode 32 is formed on a lower surface
portion exposed to the reference gas-introducing space 14, of the
lower surface of the first solid electrolyte layer 12d. An
electrochemical sensor cell, i.e., a controlling oxygen partial
pressure-detecting cell 34 is constructed by the inner pumping
electrode 24, the reference electrode 32, the second solid
electrolyte layer 12f, the second spacer layer 12e, and the first
solid electrolyte layer 12d.
[0037] The controlling oxygen partial pressure-detecting cell 34 is
operated as follows. That is, an electromotive force is generated
between the inner pumping electrode 24 and the reference electrode
32 on the basis of a difference in oxygen concentration between the
atmosphere in the first chamber 20 and the reference gas
(atmospheric air) in the reference gas-introducing space 14. The
partial pressure of oxygen in the atmosphere in the first chamber
20 can be detected by using the electromotive force.
[0038] The detected value of the partial pressure of oxygen is used
to feedback-control the variable power source 30. Specifically, the
pumping action effected by the main pumping cell 28 is controlled
by the aid of a feedback control system 36 so that the partial
pressure of oxygen in the atmosphere in the first chamber 20 has a
predetermined value which is sufficiently low to control the
partial pressure of oxygen in the second chamber 22 in the next
step.
[0039] The feedback control system 36 comprises a circuit to
perform feedback control for the pumping voltage Vp1 between the
outer pumping electrode 26 and the inner pumping electrode 24 so
that the difference (detection voltage V1) between the electric
potential of the inner pumping electrode 24 and the electric
potential of the reference electrode 32 is at a predetermined
voltage level. In this embodiment, the inner pumping electrode 24
is grounded.
[0040] Therefore, the main pumping cell 28 pumps out or pumps in
oxygen in an amount corresponding to the level of the pumping
voltage Vp1, of the measurement gas introduced into the first
chamber 20. The oxygen concentration in the first chamber 20 is
subjected to feedback control to give a predetermined level by
repeating the series of operations described above. In this state,
the pumping current Ip1, which flows between the outer pumping
electrode 26 and the inner pumping electrode 24, represents the
difference between the oxygen concentration in the measurement gas
and the controlled oxygen concentration in the first chamber 20.
The pumping current Ip1 can be used to measure the oxygen
concentration in the measurement gas.
[0041] The porous cermet electrode, which constructs each of the
inner pumping electrode 24 and the outer pumping electrode 26, is
composed of a metal such as Pt and a ceramic such as ZrO.sub.2. It
is necessary to use a material which has a weak reducing ability or
no reducing ability with respect to the NO component in the
measurement gas, for the inner pumping electrode 24 disposed in the
first chamber 20 to make contact with the measurement gas. It is
preferable that the inner pumping electrode 24 is composed of, for
example, a compound having the perovskite structure such as
La.sub.3CuO.sub.4, a cermet comprising a ceramic and a metal such
as Au having a low catalytic activity, or a cermet comprising a
ceramic, a metal of the Pt group, and a metal such as Au having a
low catalytic activity. When an alloy composed of Au and a metal of
the Pt group is used as an electrode material, it is preferable to
add Au in an amount of 0.03 to 35% by volume of the entire metal
component.
[0042] In the NOx sensor 10 according to this embodiment, a
detecting electrode 40 having a substantially rectangular planar
configuration and composed of a porous cermet electrode is formed
at a portion separated from the second diffusion rate-determining
section 18, on an upper surface portion for forming the second
chamber 22, of the upper surface of the first solid electrolyte
layer 12d. An alumina film for constructing a third diffusion
rate-determining section 42 is formed to cover the detecting
electrode 40. An electrochemical pumping cell, i.e., a measuring
pumping cell 44 is constructed by the detecting electrode 40, the
reference electrode 32, and the first solid electrolyte layer
12d.
[0043] The detecting electrode 40 is composed of a porous cermet
comprising zirconia as a ceramic and a metal capable of reducing
NOx as the measurement gas component. Accordingly, the detecting
electrode 40 functions as a NOx-reducing catalyst for reducing NOx
existing in the atmosphere in the second chamber 22. Further, the
oxygen in the atmosphere in the second chamber 22 can be pumped out
to the reference gas-introducing space 14 by applying a constant
voltage Vp2 between the detecting electrode 40 and the reference
electrode 32 by the aid of a DC power source 46. The pumping
current Ip2, which is allowed to flow in accordance with the
pumping action performed by the measuring pumping cell 44, is
detected by an ammeter 48. Details of the detecting electrode 40
will be described later on.
[0044] The constant voltage (DC) power source 46 can apply a
voltage of a magnitude to give a limiting current to the pumping
for oxygen produced during decomposition in the measuring pumping
cell 44 under the inflow of NOx restricted by the third diffusion
rate-determining section 42.
[0045] On the other hand, an auxiliary pumping electrode 50 having
a substantially rectangular planar configuration and composed of a
porous cermet electrode (for example, a cermet electrode composed
of Pt.multidot.ZrO.sub.2 containing 1% by weight of Au) is formed
on the entire lower surface portion for forming the second chamber
22, of the lower surface of the second solid electrolyte layer 12f.
An auxiliary electrochemical pumping cell, i.e., an auxiliary
pumping cell 52 is constructed by the auxiliary pumping electrode
50, the second solid electrolyte layer 12f, the second spacer layer
12e, the first solid electrolyte layer 12d, and the reference
electrode 32.
[0046] The auxiliary pumping electrode 50 is based on the use of a
material having a weak reducing ability or no reducing ability with
respect to the NO component contained in the measurement gas, in
the same manner as the inner pumping electrode 24 of the main
pumping cell 28. In this embodiment, it is preferable that the
auxiliary pumping electrode 50 is composed of, for example, a
compound having the perovskite structure such as La.sub.3CuO.sub.4,
a cermet comprising a ceramic and a metal having a low catalytic
activity such as Au, or a cermet comprising a ceramic, a metal of
the Pt group, and a metal having a low catalytic activity such as
Au. Further, when an alloy comprising Au and a metal of the Pt
group is used as an electrode material, it is preferable to add Au
in an amount of 0.03 to 35% by volume of the entire metal
components.
[0047] A desired constant voltage Vp3 is applied between the
reference electrode 32 and the auxiliary pumping electrode 50 of
the auxiliary pumping cell 52 by the aid of an external DC power
source 54. Thus, the oxygen in the atmosphere in the second chamber
22 can be pumped out to the reference gas-introducing space 14.
[0048] Accordingly, the partial pressure of oxygen in the
atmosphere in the second chamber 22 is allowed to have a low value
of partial pressure of oxygen at which the measurement of the
amount of the objective component is not substantially affected,
under the condition in which the measurement gas component (NOx) is
not substantially reduced or decomposed. In this embodiment, owing
to the operation of the main pumping cell 28 for the first chamber
20, the change in amount of oxygen introduced into the second
chamber 22 is greatly reduced as compared with the change in the
measurement gas. Accordingly, the partial pressure of oxygen in the
second chamber 22 is accurately controlled to be constant.
[0049] Therefore, in the NOx sensor 10 according to the embodiment
of the present invention constructed as described above, the
measurement gas, which has been controlled for the partial pressure
of oxygen in the second chamber 22, is introduced into the
detecting electrode 40.
[0050] As shown in FIG. 1, the NOx sensor 10 according to this
embodiment further comprises a heater 60 for generating heat in
accordance with electric power supply from the outside. The heater
60 is embedded in a form of being vertically interposed between the
first and second substrate layers 12a, 12b. The heater 60 is
provided in order to increase the conductivity of oxygen ion. An
insulative layer 62 composed of alumina or the like is formed to
cover upper and lower surfaces of the heater 60 so that the heater
60 is electrically insulated from the first and second substrate
layers 12a, 12b.
[0051] The heater 60 is arranged over the entire portion ranging
from the first chamber 20 to the second chamber 22. Accordingly,
each of the first chamber 20 and the second chamber 22 is heated to
a predetermined temperature. Simultaneously, each of the main
pumping cell 28, the controlling oxygen partial pressure-detecting
cell 34, and the measuring pumping cell 44 is also heated to a
predetermined temperature and maintained at that temperature.
[0052] Next, the operation of the NOx sensor 10 according to the
embodiment of the present invention will be explained. At first,
the forward end of the NOx sensor 10 is disposed in the external
space. Accordingly, the measurement gas is introduced into the
first chamber 20 under the predetermined diffusion resistance via
the first diffusion rate-determining section 16. The measurement
gas, which has been introduced into the first chamber 20, is
subjected to the pumping action for oxygen, caused by applying the
predetermined pumping voltage Vp1 between the outer pumping
electrode 26 and the inner pumping electrode 24 which construct the
main pumping cell 28. The partial pressure of oxygen is controlled
to have a predetermined value, for example, 10.sup.-7 atm. The
control is performed by the aid of the feedback control system
36.
[0053] The first diffusion rate-determining section 16 serves to
limit the amount of diffusion and inflow of oxygen in the
measurement gas into the measuring space (first chamber 20) when
the pumping voltage Vp1 is applied to the main pumping cell 28 so
that the current flowing through the main pumping cell 28 is
suppressed.
[0054] In the first chamber 20, a state of partial pressure of
oxygen is established, in which NOx in the atmosphere is not
reduced by the inner pumping electrode 24 in an environment of
being heated by the external measurement gas and being heated by
the heater 60. For example, a condition of partial pressure of
oxygen is formed, in which the reaction of
NO.fwdarw.1/2N.sub.2+1/2O.sub.2 does not occur, because of the
following reason. That is, if NOx in the measurement gas
(atmosphere) is reduced in the first chamber 20, it is impossible
to accurately measure NOx in the second chamber 22 disposed at the
downstream stage. In this context, it is necessary to establish a
condition in the first chamber 20 in which NOx is not reduced by
the component which participates in reduction of NOx (in this case,
the metal component of the inner pumping electrode 24).
Specifically, as described above, such a condition is achieved by
using, for the inner pumping electrode 24, the material having a
low ability to reduce NOx, for example, an alloy of Au and Pt.
[0055] The gas in the first chamber 20 is introduced into the
second chamber 22 under the predetermined diffusion resistance via
the second diffusion rate-determining section 18. The gas, which
has been introduced into the second chamber 22, is subjected to the
pumping action for oxygen, caused by applying the voltage Vp3
between the reference electrode 32 and the auxiliary pumping
electrode 50 which constitute the auxiliary pumping cell 52 to make
fine adjustment so that the partial pressure of oxygen has a
constant and low value of partial pressure of oxygen.
[0056] The second diffusion rate-determining section 18 serves to
limit the amount of diffusion and inflow of oxygen in the
measurement gas into the measuring space (second chamber 22) when
the voltage Vp3 is applied to the auxiliary pumping cell 52 so that
the pumping current Ip3 flowing through the auxiliary pumping cell
52 is suppressed, in the same manner as performed by the first
diffusion rate-determining section 16.
[0057] The measurement gas, which has been controlled for the
partial pressure of oxygen in the second chamber 22 as described
above, is introduced into the detecting electrode 40 under the
predetermined diffusion resistance via the third diffusion
rate-determining section 42.
[0058] When it is intended to control the partial pressure of
oxygen in the atmosphere in the first chamber 20 to have a low
value of the partial pressure of oxygen which does not
substantially affect the measurement of NOx, by operating the main
pumping cell 28, in other words, when the pumping voltage Vp1 of
the variable power source 30 is adjusted by the aid of the feedback
control system 36 so that the voltage V1 detected by the oxygen
partial pressure-detecting cell 34 is constant, if the oxygen
concentration in the measurement gas greatly changes, for example,
in a range of 0 to 20%, then the respective partial pressures of
oxygen in the atmosphere in the second chamber 22 and in the
atmosphere in the vicinity of the detecting electrode 40 slightly
change in ordinary cases. This phenomenon is caused probably
because of the following reason. That is, when the oxygen
concentration in the measurement gas increases, the distribution of
the oxygen concentration occurs in the widthwise direction and in
the thickness direction in the first chamber 20. The distribution
of the oxygen concentration changes depending on the oxygen
concentration in the measurement gas.
[0059] However, in the case of the NOx sensor 10 according to this
embodiment, the auxiliary pumping cell 52 is provided for the
second chamber 22 so that the partial pressure of oxygen in its
internal atmosphere always has a constant low value of the partial
pressure of oxygen. Accordingly, even when the partial pressure of
oxygen in the atmosphere introduced from the first chamber 20 into
the second chamber 22 changes depending on the oxygen concentration
in the measurement gas, the partial pressure of oxygen in the
atmosphere in the second chamber 22 can be always made to have a
constant low value, owing to the pumping action performed by the
auxiliary pumping cell 52. As a result, the partial pressure of
oxygen can be controlled to have a low value at which the
measurement of NOx is not substantially affected.
[0060] NOx in the measurement gas introduced into the detecting
electrode 40 is reduced or decomposed around the detecting
electrode 40. Thus, for example, a reaction of
NO.fwdarw.1/2N.sub.2+1/2O.sub.2 is allowed to occur. In this
process, a predetermined voltage Vp2, for example, 430 mV
(700.degree. C.) is applied between the detecting electrode 40 and
the reference electrode 32 which construct the measuring pumping
cell 44, in a direction to pump out the oxygen from the second
chamber 22 to the reference gas-introducing space 14.
[0061] Therefore, the pumping current Ip2 flowing through the
measuring pumping cell 44 has a value which is proportional to a
sum of the oxygen concentration in the atmosphere introduced into
the second chamber 22, i.e., the oxygen concentration in the second
chamber 22 and the oxygen concentration produced by reduction or
decomposition of NOx by the aid of the detecting electrode 40.
[0062] In this embodiment, the oxygen concentration in the
atmosphere in the second chamber 22 is controlled to be constant by
means of the auxiliary pumping cell 52. Accordingly, the pumping
current Ip2 flowing through the measuring pumping cell 44 is
proportional to the NOx concentration. The NOx concentration
corresponds to the amount of diffusion of NOx limited by the third
diffusion rate-determining section 42. Therefore, even when the
oxygen concentration in the measurement gas greatly changes, it is
possible to accurately measure the NOx concentration, based on the
use of the measuring pumping cell 44 by the aid of the ammeter
48.
[0063] According to the fact described above, almost all of the
pumping current value Ip2 obtained by operating the measuring
pumping cell 44 represents the amount brought about by the
reduction or decomposition of NOx. Accordingly, the obtained result
does not depend on the oxygen concentration in the measurement
gas.
[0064] The detecting electrode 40, which is formed in the second
chamber 22, will now be described in detail below. For example,
when a cermet electrode composed of Rh/ZrO.sub.2 was used for the
detecting electrode 40, a phenomenon was observed, in which the
sensitivity was lowered in accordance with the increase in
operating time.
[0065] This phenomenon was caused by the increase in impedance of
the measuring pumping cell 44. When the NOx sensor 10, in which the
impedance had been increased, was observed, it was found that the
contact area was decreased between the detecting electrode 40 and
the first solid electrolyte layer 12d. In other words, it is
postulated that the increase in impedance is caused by the decrease
in contact area between the detecting electrode 40 and the first
solid electrolyte layer 12d.
[0066] Accordingly, an experiment (conveniently referred to as
"first illustrative experiment") was carried out. In this
experiment, the way of change of the mass of the alloy composed of
Pt and Rh depending on the increase in heat was measured by using a
thermo-balance while changing the weight ratio between Pt and Rh.
Obtained results are shown in FIG. 2.
[0067] In FIG. 2, a curve "a" represents a characteristic obtained
for Pt/Rh=0/100% by weight. A curve "b" represents a characteristic
obtained for Pt/Rh=10/90% by weight. A curve "c" represents a
characteristic obtained for Pt/Rh=25/75% by weight. A curve "d"
represents a characteristic obtained for Pt/Rh=50/50% by weight. A
curve "e" represents a characteristic obtained for Pt/Rh=75/25% by
weight. A curve "f" represents a characteristic obtained for
Pt/Rh=90/10% by weight. A curve "g" represents a characteristic
obtained for Pt/Rh=100/0% by weight.
[0068] According to the experimental results shown in FIG. 2, the
following fact is understood. That is, in the case of Pt/Rh=0/100%
by weight, the increase in weight, which is caused by oxidation of
Rh (Rh.sub.2O.sub.3), is observed from about 600.degree. C. to
about 1080.degree. C. The conversion into the metal is started
again from about 1080.degree. C., and the weight is decreased. The
original weight is restored in the vicinity of about 1200.degree.
C.
[0069] Similarly, the following fact is understood. That is, in the
case of Pt/Rh=10/90% by weight, the increase in weight, which is
caused by oxidation of Rh (Rh.sub.2O.sub.3), is observed from about
700.degree. C. to about 1020.degree. C. The conversion into the
metal is started again from about 1020.degree. C., and the weight
is decreased. The original weight is restored in the vicinity of
about 1140.degree. C.
[0070] The following Table 1 summarizes the range of the increase
in weight due to the oxidation as described above, the range of the
decrease in weight due to the reconversion into the metal, and the
span of the change in weight.
1TABLE 1 Temperature Temperature range for range for Span of
increase in decrease in change in Pt/Rh weight weight weight 0/100
wt % about 600.degree. C. to about 1080.degree. C. to about 9.5 mg
about 1080.degree. C. about 1200.degree. C. 10/90 wt % about
700.degree. C. to about 1020.degree. C. to about 7.5 mg about
1020.degree. C. about 1140.degree. C. 25/75 wt % about 800.degree.
C. to about 1000.degree. C. to about 5.0 mg about 1000.degree. C.
about 1100.degree. C. 50/50 wt % about 800.degree. C. to about
960.degree. C. to about 3.5 mg about 960.degree. C. about
1070.degree. C. 75/25 wt % about 840.degree. C. to about
920.degree. C. to about 1.0 mg about 920.degree. C. about
1000.degree. C. 90/10 wt % no increase in no decrease in 0 mg
weight weight 100/0 wt % no increase in about 600.degree. C. to
about -1 mg weight about 800.degree. C.
[0071] Another illustrative experiment (conveniently referred to as
"second illustrative experiment") was carried out. In this
experiment, Pt and Rh were contained in the detecting electrode 40
in the seven ratios of Pt/Rh described above to produce NOx sensors
10 respectively. Limiting current characteristics were plotted for
the seven types of NOx sensors 10. FIG. 3 shows the limiting
current characteristics obtained by applying the heating operation
in the atmospheric air at 800.degree. C. FIG. 4 shows the limiting
current characteristics obtained by applying the heating operation
in the atmospheric air at 700.degree. C.
[0072] As for the characteristics shown in FIG. 3, an abnormal
pumping current having a peak of about 20 mA flew in the NOx sensor
10 with Pt/Rh=0/100% by weight. An abnormal pumping current having
a peak of about 18 mA flew in the NOx sensor 10 with Pt/Rh=10/90%
by weight. An abnormal pumping current having a peak of about 15 mA
flew in the NOx sensors 10 with Pt/Rh=25/75% by weight and
Pt/Rh=50/50% by weight respectively. An abnormal pumping current
having a peak of about 4 mA flew in the NOx sensor 10 with
Pt/Rh=75/25% by weight.
[0073] That is, as shown in FIG. 3, when the heating operation was
applied in the atmospheric air at 800.degree. C., the abnormal
increase in pumping current was observed at a rate corresponding to
the magnitude of the peak value of the increment of the weight
shown in FIG. 2. When the heating operation was applied in the
atmospheric air at 700.degree. C., the abnormal increase in pumping
current was observed only for the NOx sensor 10 having the
detecting electrode 40 of Pt/Rh=0/100% by weight. The abnormal
increase in pumping current was not observed for the other NOx
sensors 10. According to this fact, it is considered that the
abnormal increase in pumping current shown in FIGS. 3 and 4 is the
increase caused by the pumping action for oxygen originating from
the oxide of Rh (Rh.sub.2O.sub.3).
[0074] When the NOx sensor 10 is practically used, the element
temperature is usually set to be about 700.degree. C. Therefore,
for example, when the detecting electrode 40 is constructed by a
cermet electrode of Rh=100% by weight, the decrease in volume
occurs due to the reconversion into the metal of Rh by the oxygen
pumping effected by the detecting electrode 40 during the operation
of the sensor. Immediately after the operation of the sensor is
stopped, the oxygen pumping is stopped, but the element temperature
is still not less than 600.degree. C. Therefore, the oxidation of
Rh (Rh.sub.2O.sub.3) occurs, and the increase in volume of Rh takes
place.
[0075] The contact area between the detecting electrode 40 and the
first solid electrolyte layer 12d is decreased by the repetition of
the series of the increase in volume and the decrease in volume. In
such a situation, it is postulated that the impedance of the
measuring pumping cell 44 is increased, and the sensitivity to NOx
is decreased.
[0076] A still another illustrative experiment (conveniently
referred to as "third illustrative experiment") will now be
described. In the third illustrative experiment, observation was
made for the change in sensitivity to NOx upon practical use of NOx
sensors concerning Working Example and Comparative Example. In
Working Example, the ratio of Pt/Rh contained in the detecting
electrode 40 was 50/50% by weight for the NOx sensor 10 according
to the embodiment of the present invention. In Comparative Example,
the ratio of Pt/Rh contained in the detecting electrode 40 was
0/100% by weight. Results of the third illustrative experiment are
shown in FIGS. 5A and 5B. In these drawings, the characteristic
obtained in Working Example is depicted by a solid line, and the
characteristic obtained in Comparative Example is depicted by a
broken line.
[0077] According to the experimental results, the increase in
impedance of the measuring pumping cell 44 was started from about
1800 hours in Comparative Example, and the decrease in sensitivity
to NOx was observed in accordance with the increase in impedance.
On the other hand, in Working Example, no increase in impedance was
observed even after the passage of 4000 hours, and no decrease in
sensitivity to NOx was observed as well.
[0078] As described above, in the NOx sensor 10 according to the
embodiment of the present invention, the cermet electrode, which is
composed of the Pt--Rh alloy and the ceramic component, is used as
the detecting electrode 40 for constructing the measuring pumping
cell 44. Therefore, it is possible to suppress the oxidation of Rh
and the reconversion into the metal contained in the detecting
electrode 40. Even when the operating time of the NOx sensor 10 is
increased, there is no occurrence of the increase in impedance
which would be otherwise caused by the decrease in contact area
between the detecting electrode 40 and the first solid electrolyte
layer 12d. That is, the NOx sensor 10 according to the embodiment
of the present invention makes it possible to stabilize the
impedance and stabilize the measurement sensitivity.
[0079] As also understood from the experimental results described
above, the ratio between Pt and Rh contained in the detecting
electrode 40 is preferably, in weight ratio, Pt:Rh=10:90 to 90:10,
and more preferably Pt:Rh=25:75 to 75:25.
[0080] Next, a modified embodiment 10a of the NOx sensor 10
according to the foregoing embodiment will be described with
reference to FIG. 6. Components or parts corresponding to those
shown in FIG. 1 are designated by the same reference numerals.
[0081] As shown in FIG. 6, a NOx sensor 10a according to the
modified embodiment is constructed in approximately the same manner
as the NOx sensor 10 according to the foregoing embodiment (see
FIG. 1). However, the former is different from the latter in that a
measuring oxygen partial pressure-detecting cell 70 is provided in
place of the measuring pumping cell 44.
[0082] The measuring oxygen partial pressure-detecting cell 70
comprises a detecting electrode 72 formed on an upper surface
portion for forming the second chamber 22, of the upper surface of
the first solid electrolyte layer 12d, the reference electrode 32
formed on the lower surface of the first solid electrolyte layer
12d, and the first solid electrolyte layer 12d interposed between
the both electrodes 72, 32.
[0083] In this embodiment, an electromotive force (electromotive
force of an oxygen concentration cell) V2 corresponding to the
difference in oxygen concentration between the atmosphere around
the detecting electrode 72 and the atmosphere around the reference
electrode 32 is generated between the reference electrode 32 and
the detecting electrode 72 of the measuring oxygen partial
pressure-detecting cell 70.
[0084] Therefore, the partial pressure of oxygen in the atmosphere
around the detecting electrode 72, in other words, the partial
pressure of oxygen defined by oxygen produced by reduction or
decomposition of the measurement gas component (NOx) is detected as
a voltage value by measuring the electromotive force (voltage V2)
generated between the detecting electrode 72 and the reference
electrode 32 by using a voltmeter 74.
[0085] Also in the NOx sensor 10a according to the modified
embodiment, the cermet electrode, which is composed of the Pt--Rh
alloy and the ceramic component, is used as the detecting electrode
72 for constructing the measuring oxygen partial pressure-detecting
cell 70. As a result, it is possible to suppress the oxidation of
Rh and the reconversion into the metal contained in the detecting
electrode 72. Even when the operating time of the NOx sensor 10a is
increased, there is no occurrence of the increase in impedance
which would be otherwise caused by the decrease in contact area
between the detecting electrode 72 and the first solid electrolyte
layer 12d. That is, the NOx sensor 10a according to the modified
embodiment also makes it possible to stabilize the impedance and
stabilize the measurement sensitivity.
[0086] It is a matter of course that the NOx
concentration-measuring apparatus according to the present
invention is not limited to the embodiments described above, which
may be embodied in other various formed without deviating from the
gist or essential characteristics of the present invention.
[0087] As explained above, according to NOx concentration-measuring
apparatus concerning the present invention, it is possible to
suppress the oxidation of Rh and the reconversion into the metal
contained in the NOx-decomposing electrode, stabilize the
impedance, and stabilize the measurement sensitivity.
* * * * *