Title: Gas sensing element and method for manufacturing the same
Abstract: A gas sensing element has a solid electrolytic body, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. A crystal face strength ratio of the measured gas side electrode according to X-ray diffraction is 0.7.ltoreq.{I(200)/I(111)} or 0.6.ltoreq.{I(220)/I(111)}.
Patent Number: 6,849,291 Issued on 02/01/2005 to E, et al.
| Inventors:
|
E; Gang (Anjo, JP);
Kobayashi; Kiyomi (Kuwana, JP);
Hotta; Yasumichi (Mie-ken, JP);
Fujii; Namitsugu (Yokkaichi, JP)
|
| Assignee:
|
Denso Corporation (Kariya, JP)
|
| Appl. No.:
|
271534 |
| Filed:
|
October 17, 2002 |
Foreign Application Priority Data
| Aug 07, 2000[JP] | 2000-238723 |
| May 01, 2001[JP] | 2001-134423 |
| Current U.S. Class: |
427/58; 427/123; 427/126.5; 427/304; 427/372.2; 427/443.1; 427/532; 427/554 |
| Intern'l Class: |
B05D 005/12 |
| Field of Search: |
427/58,123,126.5,304,372.2,443.1,532,554
|
References Cited [Referenced By]
U.S. Patent Documents
| 4107018 | Aug., 1978 | Bode et al. | 204/427.
|
| 4265724 | May., 1981 | Haecker et al. | 204/429.
|
| 4650697 | Mar., 1987 | Kitagawa et al.
| |
| 4828673 | May., 1989 | Maeda | 204/427.
|
| 5077270 | Dec., 1991 | Takeda et al.
| |
| 5099172 | Mar., 1992 | Taguchi et al.
| |
| 5202154 | Apr., 1993 | Matsuura et al.
| |
| 5512151 | Apr., 1996 | Hayamizu et al.
| |
| 5767036 | Jun., 1998 | Freund et al. | 502/185.
|
| 6096372 | Aug., 2000 | Nomura et al.
| |
| 6254926 | Jul., 2001 | Katafuchi et al. | 427/125.
|
| 6326098 | Dec., 2001 | Itoh et al.
| |
| Foreign Patent Documents |
| 0513821 | Nov., 1992 | EP.
| |
| 0686847 | Dec., 1995 | EP.
| |
| 2-85756 | Mar., 1990 | JP.
| |
| 5-77264 | Oct., 1993 | JP.
| |
| 10-104194 | Apr., 1998 | JP.
| |
Other References
Patent Abstracts of Japan, vol. 013, No. 032 (P-817), Jan. 25, 1989 & JP 63
231255 A (NGK Spark Plug Co Ltd), Sep. 27, 1988.
|
Primary Examiner: Talbot; Brian K.
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Parent Case Text
This application is a Division of Ser. No. 09/922,716 filed Aug. 7, 2001
now U.S. Pat. No. 6,478,941.
Claims
What is claimed is:
1. A method of manufacturing a gas sensing element comprising a solid
electrolytic body, a reference gas side electrode provided on a surface of
said solid electrolytic body so as to be exposed to a reference gas, and a
measured gas side electrode provided on another surface of said solid
electrolytic body so as to be exposed to a measured gas, said
manufacturing method comprising:
providing a fine-grain nucleus of noble metal on an electrode forming
portion of said solid electrolytic body;
applying a reducing heat treatment to said fine-grain nucleus;
forming an unbaked film electrode on said fine-grain nucleus; and
baking said film electrode in a reducing atmosphere to form said measured
gas side electrode thereby providing a crystal face strength ratio of said
measured gas side electrode according to X-ray diffraction satisfies the
following relationship
0.7.ltoreq.{I(200)/I(111)} or 0.61.ltoreq.{I(220)/I(111)}
where I(200) represents the intensity of an X-ray diffraction measurement
for a crystal face (200) of said measured gas side electrode, I(111)
represents the intensity of an X-ray diffraction measurement for a crystal
face (111) of said measured gas side electrode and I(220) represents the
intensity of an X-ray diffraction measurement for a crystal face (220) of
said measured gas side electrode.
2. A method of manufacturing a gas sensing element comprising a solid
electrolytic body, a reference gas side electrode provided on a surface of
said solid electrolytic body so as to be exposed to a reference gas, and a
measured gas side electrode provided on another surface of said solid
electrolytic body so as to be exposed to a measured gas, said
manufacturing method comprising:
providing a fine-grain nucleus of noble metal on an electrode forming
portion of said solid electrolytic body;
irradiating a laser beam to said fine-grain nucleus;
forming an unbaked film electrode on said fine-grain nucleus; and
baking said film electrode in a reducing atmosphere to form said measured
gas side electrode thereby providing a crystal face strength ratio of said
measured gas side electrode according to X-ray diffraction satisfies the
following relationship
0.7.ltoreq.{I(200)/I(111)} or 0.6.ltoreq.{I(220)/I(111)}
where I(200) represents the intensity of an X-ray diffraction measurement
for a crystal face (200) of said measured gas side electrode, I(111)
represents the intensity of an X-ray diffraction measurement for a crystal
face (111) of said measured gas side electrode and I(220) represents the
intensity of an X-ray diffraction measurement for a crystal face (220) of
said measured gas side electrode.
3. A method of manufacturing a gas sensing element comprising a solid
electrolytic body, a reference gas side electrode provided on a surface of
said solid electrolytic body so as to be exposed to a reference gas, and a
measured gas side electrode provided on another surface of said solid
electrolytic body so as to be exposed to a measured gas, said
manufacturing method comprising:
preparing a paste of fine-grain nucleus of noble metal applied crystal face
orientation processing beforehand;
forming an unbaked film electrode by coating said paste on an electrode
forming portion of said solid electrolytic body; and
baking said film electrode in a reducing atmosphere to form said measured
gas side electrode thereby providing a crystal face strength ratio of said
measured gas side electrode according to X-ray diffraction satisfies the
following relationship
0.7.ltoreq.{I(200)/I(111)} or 0.6.ltoreq.{I(220)/I(111)}
where I(200) represents the intensity of an X-ray diffraction measurement
for a crystal face (200) of said measured gas side electrode, I(111)
represents the intensity of an X-ray diffraction measurement for a crystal
face (111) of said measured gas side electrode and I(220) represents the
intensity of an X-ray diffraction measurement for a crystal face (220) of
said measured gas side electrode.
4. A method of manufacturing a gas sensing element comprising a solid
electrolytic body, a reference gas side electrode provided on a surface of
said solid electrolytic body so as to be exposed to a reference gas, and a
measured gas side electrode provided on another surface of said solid
electrolytic body so as to be exposed to a measured gas, said
manufacturing method comprising:
providing a fine-grain nucleus of noble metal on an electrode forming
portion of said solid electrolytic body;
applying a reducing heat treatment to said fine-grain nucleus;
forming an unbaked film electrode on said fine-grain nucleus; and
baking said film electrode in a reducing atmosphere to form said measured
gas side electrode thereby providing a crystal face strength ratio of said
measured gas side electrode according to X-ray diffraction satisfies the
following relationship
1.3.ltoreq.{I(200)+I(200)}/I(111)
where I(200) represents the intensity of an X-ray diffraction measurement
for a crystal face (200) of said measured gas side electrode, I(220)
represents the intensity of an X-ray diffraction measurement for a crystal
face (220) of said measured gas side electrode, and I(111) represents the
intensity of an X-ray diffraction measurement for a crystal face (111) of
said measured gas side electrode.
5. A method of manufacturing a gas sensing element comprising a solid
electrolytic body, a reference gas side electrode provided on a surface of
said solid electrolytic body so as to be exposed to a reference gas, and a
measured gas side electrode provided on another surface of said solid
electrolytic body so as to be exposed to a measured gas, said
manufacturing method comprising:
providing a fine-grain nucleus of noble metal on an electrode forming
portion of said solid electrolytic body;
irradiating a laser beam to said fine-grain nucleus;
forming an unbaked film electrode on said fine-grain nucleus; and
baking said film electrode in a reducing atmosphere to form said measured
gas side electrode thereby providing a crystal face strength ratio of said
measured gas side electrode according to X-ray diffraction satisfies the
following relationship
1.3.ltoreq.{I(200)+I(200)}/I(111)
where I(200) represents the intensity of an X-ray diffraction measurement
for a crystal face (200) of said measured gas side electrode, I(220)
represents the intensity of an X-ray diffraction measurement for a crystal
face (220) of said measured gas side electrode, and I(111) represents the
intensity of an X-ray diffraction measurement for a crystal face (111) of
said measured gas side electrode.
6. A method of manufacturing a gas sensing element comprising a solid
electrolytic body, a reference gas side electrode provided on a surface of
said solid electrolytic body so as to be exposed to a reference gas, and a
measured gas side electrode provided on another surface of said solid
electrolytic body so as to be exposed to a measured gas, said
manufacturing method comprising:
preparing a paste of fine-grain nucleus of noble metal applied crystal face
orientation processing beforehand;
forming an unbaked film electrode by coating said paste on an electrode
forming portion of said solid electrolytic body; and
baking said film electrode in a reducing atmosphere to form said measured
gas side electrode thereby providing a crystal face strength ratio of said
measured gas side electrode according to X-ray diffraction satisfies the
following relationship
1.3.ltoreq.{I(200)+I(220)}/I(111)
where I(200) represents the intensity of an X-ray diffraction measurement
for a crystal face (200) of said measured gas side electrode, I(220)
represents the intensity of an X-ray diffraction measurement for a crystal
face (220) of said measured gas side electrode, and I(111) represents the
intensity of an X-ray diffraction measurement for a crystal face (111) of
said measured gas side electrode.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensor installed in an exhaust
system of an automotive internal combustion engine to detect an oxygen
concentration in the exhaust gas, or an air-fuel ratio, or the like.
The present invention relates to a gas sensing element used for controlling
an air-fuel ratio of an internal combustion engine and a method for
manufacturing the gas sensing element.
In general, to control the air-fuel ratio, a gas sensor is installed in an
exhaust system of an automotive internal combustion engine.
The gas sensor comprises a gas sensing element provided at its front end
for detecting an oxygen concentration. The gas sensing element comprises a
solid electrolytic sintered body having oxygen ion conductance, a
reference gas side electrode provided on a surface of the solid
electrolytic body so as to be exposed to a reference gas, and a measured
gas side electrode provided on another surface of the solid electrolytic
body so as to be exposed to a measured gas. The measured gas side
electrode is covered by a porous electrode protective layer.
In many cases, the electrode protective layer is a ceramic coating layer,
or a double layer consisting of a ceramic coating layer and a
.gamma.-Al2O3 layer provided on this ceramic coating layer.
According to this type of gas sensing element, a measured gas reaches a
measured gas side electrode through the ceramic coating layer or the
double layer of the ceramic coating layer and the .gamma.-A1203 layer. The
bas sensing element produces a sensor output.
Recent radically changing circumstances, such as enhancement of emission
control laws and regulations as well as requirements for high power
internal combustion engines, has forced automotive manufacturers to
develop automotive engines capable of precisely controlling combustion.
To realize this, it is essentially important to provide excellent gas
sensors having sensing properties stable under severe operating conditions
and durable for a long-term use.
FIG. 5 shows a characteristic curve representing a relationship between
air-fuel ratio and voltage, as important sensor output characteristics of
a gas sensing element used for combustion control of an internal
combustion engine. In FIG. 5, point .lambda. is referred to as a specific
air-fuel ratio where the voltage undergoes steep changes. In FIG. 5, a
reference voltage is a criteria used for judging whether a fuel injection
amount should be increased or decreased in the combustion control of an
internal combustion engine. In general, the reference voltage is set to
0.45V.
More specifically, when a sensor output is larger than the reference
voltage, the fuel injection amount is reduced to form an air/fuel mixture
whose air-fuel ratio is shifted to a lean side. On the contrary, when a
sensor output is less than the reference voltage, the fuel injection
amount is increased to form a relatively rich air/fuel mixture. Through
such a feedback control, the air-fuel ratio of the controlled engine can
be always kept in a window of a ternary catalyst.
Accordingly, to precisely perform the air-fuel ratio control, it is
essentially important to stabilize the point .lambda. (hereinafter,
referred to as control .lambda.).
In other words, the control .lambda. should be stable during a long-term
use of a gas sensing element and should be constant regardless of any
environmental change of the gas sensing element.
When a gas sensing element is installed in an exhaust system of an internal
combustion engine, a sensor output is produced in the following manner.
First, an exhaust gas containing unburnt components reaches a measured gas
side electrode. Then, an equilibrium oxygen concentration is obtained
through a catalytic reaction caused on the measured gas side electrode.
The sensor output is produced as a signal representing a difference
between the equilibrium oxygen concentration thus obtained and an oxygen
concentration in the air serving as a reference gas.
Accordingly, it becomes possible to increase the measuring accuracy of a
gas sensing element when an electrode having excellent activity is used as
a measured gas side electrode of a gas sensing element.
The following is a method for activating a measured gas side electrode
disclosed in Unexamined Japanese patent publication No. 10-104194.
First, a measured gas side electrode is formed on a surface of a solid
electrolytic body by baking in the air at the temperature range from
1,000.degree. C. to 1,400.degree. C. Then, a heat treatment is applied to
the measured gas side electrode thus formed in an atmosphere containing
H.sub.2.
Subsequently, a heat treatment in an inert atmosphere and a heat treatment
in a non-oxidative atmosphere including moisture vapor are applied to the
measured gas side electrode.
By combining these treatments, the catalytic activity of the measured gas
side electrode can be enhanced.
However, according to the above-described conventional method, it was
difficult to provide a gas sensing element having a measured gas side
electrode which can assure a sufficiently stable control .lambda. even in
a severe high-temperature environment or in a poisonous environment
containing Si compounds.
SUMMARY OF THE INVENTION
In view of the foregoing problems of the prior art, the present invention
has an object to provide a gas sensing element capable of demonstrating
excellent performances high temperature and/or Si poisoning environments.
To accomplish the above and other related objects, the present invention
provides a first gas sensing element comprising a solid electrolytic body,
a reference gas side electrode provided on a surface of the solid
electrolytic body so as to be exposed to a reference gas, and a measured
gas side electrode provided on another surface of the solid electrolytic
body so as to be exposed to a measured gas, wherein a crystal face
strength ratio of the measured gas side electrode according to X-ray
diffraction is 0.7.ltoreq.{I(200)/I(111)} or 0.6.ltoreq.{I(220)/I(111)}.
The first gas sensing element of the present invention is characterized in
that the measured gas side electrode has a crystal face strength ratio
according to X-ray diffraction satisfying the above-described conditions.
If I(200)/I(111) is less than 0.7, a ratio of an active surface to an
entire electrode surface will reduce to 0.5 or less and it will be
difficult to assure satisfactory catalytic activity and stability for
smoothly promoting an equilibrating reaction of exhaust gas.
If I(220)/I(111) is less than 0.6, it will be difficult to assure
satisfactory catalytic activity and stability.
To obtain crystal grains and an electrode film which are stable in energy
level and easily fabricable, a preferable upper limit of the crystal face
strength ratio is 1.0 in view of the fact that the total area of active
faces (200) and (220) can be maximized and because according to this
condition the crystal face orientation can satisfy the requirement that
the solid electrolytic body causes no alteration.
Next, functions and effects of the present invention will be explained
hereinafter.
Inventors of this invention enthusiastically conducted research and
development for stabilizing the activity of a measured gas side electrode,
i.e., stabilization of control .lambda.. And, as a result of the research
and development, the inventors have found the fact that a crystal face of
the measured gas side electrode greatly contributes to activation and
stability of a gas sensing element.
The measured gas side electrode is made of an electrode material containing
noble metals which possess catalytic properties and usually have a
face-centered cubic structure.
In a crystal lattice of such metals, specific crystal faces, i.e., faces
(100) and (110), have a lower surface density of atoms compared with the
other face (111) dominant in this crystal lattice.
Due to lower surface densities, these faces (100) and (110) promote
adsorption of various exhaust components. Thus, these crystal faces can
smoothly adsorb unburnt exhaust components and residual oxygen when the
measured gas is exhaust gas. The equilibrating reaction smoothly advances.
According to the measured gas side electrode satisfying the above-described
requirements, the faces (100) and (110) are orientated on the surface of
the measured gas side electrode.
In the X-ray diffraction of a noble metal having a face-centered cubic
structure, both faces (100) and (110) appear as faces (200) and (220)
respectively. Hence, the strength ratio of this invention is expressed by
using faces (200) and (220). Namely, strengths of faces (100) and (110)
can be replaced by those of faces (200) and (220). The same result is
obtained.
As described above, according to the gas sensing element of this invention,
the entire surface of the measured gas side electrode possesses higher
activity. Even when the measured gas side electrode is exposed to a severe
high-temperature environment or in a poisoning environment containing Si
components, the equilibrating reaction of exhaust gas can advance smoothly
on the electrode surface. In this respect, the measured gas side electrode
of this invention possesses excellent catalytic properties. Thus, it
becomes possible to provides a sensor whose output is stable even after a
long-term use in the high-temperature environment and which is robust
against Si poisoning.
Accordingly, the gas sensing element of the present invention can maintain
a stable control .lambda. for a long time.
As apparent from the foregoing description, the present invention can
provide a gas sensing element capable of demonstrating excellent
performances in heat resistivity as well as in Si poisoning durability.
Furthermore, the present invention provides a second gas sensing element
comprising a solid electrolytic body, a reference gas side electrode
provided on a surface of the solid electrolytic body so as to be exposed
to a reference gas, and a measured gas side electrode provided on another
surface of the solid electrolytic body so as to be exposed to a measured
gas, wherein a crystal face strength ratio of the measured gas side
electrode according to X-ray diffraction is
1.3.ltoreq.{I(200)+I(220)}/I(111).
The measured gas side electrode of this gas sensing element has a crystal
face strength ratio according to X-ray diffraction satisfying the
above-described conditions.
If the crystal face strength ratio is less than 1.3, activity of the
measured gas side electrode will be soon worsened in severe operating
conditions, such as a high-temperature environment and a Si poisoning
environment. The control .lambda. will vary widely depending on the
operating conditions. Thus, the gas concentration cannot be measured
accurately.
To obtain crystal grains and an electrode film which are stable in energy
level and fabricable, a preferable upper limit of {I(200)+I(220)}/I(111)
is 2.0 in view of the fact that the total area of faces (200) and (220)
can be maximized and because this condition satisfies crystal face
orientation requirement that the solid electrolytic body causes no
alteration.
As described above, the measured gas side electrode is made of an electrode
material containing noble metals which possess catalytic properties and
usually have a face-centered cubic structure.
In a crystal lattice of such metals, both faces (100) and (110) have a
lower surface density of atoms compared with other face (111) dominant in
this crystal lattice.
Accordingly, these faces (100) and (110) promote adsorption of various
exhaust components and act as active faces smoothly advancing the
equilibrating reaction.
According to the measured gas side electrode satisfying the requirement
1.3.ltoreq.{I(200)+I(220)}/I(111), the above-described active faces are
orientated on the surface of the measured gas side electrode. Thus, it
becomes possible to provides a sensor whose output is stable even after a
long-term use in the high-temperature environment and robust against Si
poisoning. Accordingly, the base sensing element of the present invention
can maintain a stable control .lambda. for a long time.
As apparent from the foregoing description, the present invention can
provide a gas sensing element capable of demonstrating excellent
performances in heat resistivity as well as in Si poisoning durability.
Furthermore, it is preferable that {I(200)+I(220)}/I(111) is equal to or
larger than 1.5.
With this arrangement, it becomes possible to easily form an electrode
having a uniform film thickness and possessing stable and excellent
catalytic activity without causing alteration of a solid electrolytic
body.
The strength ratio of the above-described crystal faces can be obtained by
measuring the X-ray diffraction strength of a surface of the measured gas
side electrode (to be exposed to a measured gas) according to the X-ray
diffraction method, for example by using a position sensitive proportional
counter (PSPC) type microdiffractometer, manufactured by Rigaku
Corporation. Alternatively, the crystal face strength ratio can be
obtained according to another X-ray diffraction method using a similar
diffractometer.
The following is an example of practical measurement.
According to the X-ray diffraction method using the above-described
measuring apparatus, a thin X-ray with a diameter in the range from 200
.mu.m to 300 .mu.m with a power of 40 kV and 80 mA is irradiated onto a
surface of the measured gas side electrode of each tested element piece
(whose size is equal to or less than 5 mm) fixed by a sample holder.
Then, the strength ratio of crystal faces is calculated based on data
collected simultaneously by a PSPC program (capable of performing both
measurement and data processing) manufactured by Rigaku Corporation, while
X-ray diffraction angle (2.theta.) varies in the range from 20.degree. to
80.degree.. This measurement is performed in the air at a room
temperature. The measured data is subjected to correction based on a Si
powder standard sample.
As described above, in the X-ray diffraction of a noble metal having a
face-centered cubic structure, both faces (100) and (110) appear as faces
(200) and (220) respectively. Hence, strengths of faces (100) and (110)
can be replaced by those of faces (200) and (220) respectively.
When the noble metal having a face-centered cubic structure is random and
does not have specific orientation, the strength ratio
{I(200)+I(220)}/I(111) becomes 0.84.
Furthermore, the present invention provides a first method of manufacturing
a gas sensing element comprising a solid electrolytic body, a reference
gas side electrode provided on a surface of the solid electrolytic body so
as to be exposed to a reference gas, and a measured gas side electrode
provided on another surface of the solid electrolytic body so as to be
exposed to a measured gas. The first manufacturing method comprises a step
of providing a fine-grain nucleus (or a particulate core) of noble metal
on an electrode forming portion of the solid electrolytic body, a step of
applying a reducing heat treatment to the fine-grain nucleus, a step of
forming a unbaked film electrode on the fine-grain nucleus, and a step of
baking the film electrode in a reducing atmosphere to form the measured
gas side electrode.
By utilizing the first manufacturing method of this invention, it becomes
possible to form a measured gas side electrode from a fine-grain nucleus
having active faces, such as faces (100) and (110), having lower surface
densities of atoms.
According to the first manufacturing method, a fine-grain nucleus of noble
metal is provided on an electrode forming portion (i.e., a portion where a
measured gas side electrode is to be provided) of a solid electrolytic
body. Then, a reducing heat treatment is applied to the fine-grain
nucleus.
Grains of noble metal have higher surface energy. Applying a reducing heat
treatment to a fine-grain nucleus induces chemical adsorption of reducing
gas molecules in addition to thermal action.
As a result, active crystal faces having higher energy level, i.e., faces
(100) and (110) having lower atomic surface densities, are formed on the
fine-grain nucleus.
In a reducing atmosphere, the above-described active faces chiefly adsorb
the molecules having strong reducing properties and higher activity, such
as hydrogen molecules and carbon monoxide molecules. Growth of these
active faces becomes slow compared with that of the other crystal face. As
a result, the percentage of a crystal face (111) having a higher growth
rate reduces and the active faces having slow growth rates remain in a
wide region.
Therefore, in a case where an unbaked electrode film is formed on the
fine-grain nucleus having active faces and this electrode film is baked in
a reducing atmosphere, the orientation of the active faces can be
maintained during the growth of the fine-grain nucleus. Thus, the
electrode can be formed.
Accordingly, it becomes possible to obtain a gas sensing element having a
measured gas side electrode on the surface of which active faces are
orientated. The measured gas side electrode, when the active faces are
dominant on the surface thereof, can produce a stable output even after a
long-term use in the above-described high-temperature environment and can
possess preferable durability against Si poisoning. Accordingly, the gas
sensing element of the present invention can maintain a stable control
.lambda. for a long time.
As apparent from the foregoing description, the present invention can
provide a method for manufacturing a gas sensing element capable of
demonstrating excellent performances in heat resistivity as well as in Si
poisoning durability.
The above-described electrode film can be formed by utilizing chemical
(i.e., electroless) plating, sputtering, vaporization etc.
A material can be used as the fine-grain nucleus when it includes at least
one noble metal having a face-centered cubic structure such as Pt, Rh, Ir,
Pd, Au etc.
The first manufacturing method of this invention can be put into practice
in the following manner. First, a Pt nucleus is formed on a surface of a
solid electrolytic body by reducing a chloroplatinic acid solution. Then,
the Pt nucleus is subjected to the reducing thermal processing in a
reducing atmosphere at a high temperature. Then, an electroless plating is
applied to the Pt nucleus, thereby forming a plating layer on the Pt
nucleus. Thereafter, the plating layer is thermally treated in a reducing
atmosphere to form a measured gas side electrode.
It is preferable that the reducing heat treatment for the fine-grain
nucleus is performed in the temperature range from 600.degree. C. to
800.degree. C.
If the temperature is less than 600.degree. C., the effect of orientating
the active faces on the fine-grain nucleus will be reduced. If the
temperature is higher than 800.degree. C., mutual bonding and grain growth
will be caused between fine-grain nucleuses. No fine-grain nucleus may be
formed on the electrode forming portion of a solid electrolytic body. The
electrode film cannot be formed in an intended manner.
Furthermore, H.sub.2 --N.sub.2 series atmosphere can be preferably used for
the reducing heat treatment applied to the fine-grain nucleus according to
this invention.
Hydrogen atoms are selectively adsorbed on faces (100) and (110) of the
crystal lattice constituting the fine-grain nucleus. This promotes the
formation of active faces.
Furthermore, it is preferable that the concentration of H.sub.2 contained
in the reducing heat treatment atmosphere is equal to or larger than 5 vol
%.
If the H.sub.2 concentration is less than 5 vol %, the active faces may not
be formed due to lack of H.sub.2 in the reducing heat treatment
atmosphere.
Furthermore, the present invention provides a second method of
manufacturing a gas sensing element comprising a solid electrolytic body,
a reference gas side electrode provided on a surface of the solid
electrolytic body so as to be exposed to a reference gas, and a measured
gas side electrode provided on another surface of the solid electrolytic
body so as to be exposed to a measured gas. The second manufacturing
method comprises a step of providing a fine-grain nucleus of noble metal
on an electrode forming portion of the solid electrolytic body, a step of
irradiating a laser beam to the fine-grain nucleus, a step of forming a
unbaked electrode film on the fine-grain nucleus, and a step of baking the
electrode film in a reducing atmosphere to form the measured gas side
electrode.
Like the above-described reducing heat treatment, performing the laser
irradiating processing is effective to activate and rearrange the
surficial atoms of the fine-grain nucleus. Accordingly, it becomes
possible to obtain a gas sensing element having a measured gas side
electrode on the surface of which active faces are orientated. The
measured gas side electrode, when the active faces are dominant on the
surface thereof, can produce a stable output even after a long-term use in
the above-described high-temperature environment and can possess
preferable durability against Si poisoning. Accordingly, the gas sensing
element of the present invention can maintain a stable control .lambda.
for a long time.
As apparent from the foregoing description, the present invention can
provide a method for manufacturing a gas sensing element capable of
demonstrating excellent performances in heat resistivity as well as in Si
poisoning durability.
The second manufacturing method of this invention can be put into practice
in the following manner. First, a Pt nucleus is formed by reducing a
chloroplatinic acid solution and is subjected to the laser irradiation
processing. Thereafter, an electroless plating is applied to the Pt
nucleus, thereby forming a plating layer comprising a Pt grain polycrystal
having excellent orientation.
Then, the Pt plating later is baked in an inert atmosphere to obtain a
measured gas side electrode having excellent orientation.
Regarding the laser irradiation, a preferable laser power is in the range
from 10 mW to 50 mW and a preferable irradiation time is in the range from
1 minute to 30 minutes.
When the laser irradiation is performed within the above irradiation time,
increasing the laser power up to 50 mW will give a significant damage to a
solid electrolytic body and accordingly will cause local alterations on
the solid electrolytic body (i.e., black spots).
Furthermore, if the irradiation time exceeds 30 minutes, the fine-grain
nucleus will agglutinate or partly evaporate. The electrode film thus
formed will not have a uniform film thickness.
If the laser power is less than 10 mW, or if the irradiation time is
shorter than 1 minute, no orientation will be caused on the fine-grain
nucleus. The effects of the present invention will not be obtained.
Furthermore, the present invention provides a third method of manufacturing
a gas sensing element comprising a solid electrolytic body, a reference
gas side electrode provided on a surface of the solid electrolytic body so
as to be exposed to a reference gas, and a measured gas side electrode
provided on another surface of the solid electrolytic body so as to be
exposed to a measured gas. The third manufacturing method comprises a step
of preparing a paste of fine-grain nucleus of noble metal applied crystal
face orientation processing beforehand, a step of forming a unbaked
electrode film by coating the paste on an electrode forming portion of the
solid electrolytic body, and a step of baking the electrode film in a
reducing atmosphere to form the measured gas side electrode.
Orientating crystal faces of the fine-grain nucleus of noble metal makes it
possible to form the active faces, i.e., faces (100) and (110), having
higher energy levels.
According to the present invention, a paste is prepared so as to include
the fine-grain nucleus of noble metal applied to the crystal face
orientation processing beforehand. Then, an electrode film is formed by
using this paste. Then, a reducing heat treatment is applied. The
fine-grain nucleus grows while maintaining the orientation of the active
faces.
Accordingly, as the electrode film is formed and baked while maintaining
the orientation adequately, it becomes possible to obtain a gas sensing
element having a measured gas side electrode on the surface of which
active faces are orientated.
As described above, the measured gas side electrode can produce a stable
output even after a long-term use in the above-described high-temperature
environment and can possess preferable durability against Si poisoning.
Accordingly, the gas sensing element of the present invention can maintain
a stable control .lambda. for a long time.
As apparent from the foregoing description, the present invention can
provide a method for manufacturing a gas sensing element capable of
demonstrating excellent performances in heat resistivity as well as in Si
poisoning durability.
The third manufacturing method of this invention can be put into practice
in the following manner.
First, a chloroplatinic acid is thermally decomposed in an inert atmosphere
at the temperature range from 1,000.degree. C. to 1,100.degree. C. to
obtain a Pt nucleus with active faces having excellent orientation. The Pt
nucleus thus obtained is mixed with a binder and the like to form a paste.
Then, the paste is baked in an inert gas atmosphere to obtain a measured
gas side electrode whose active faces have excellent orientation.
Furthermore, in any of the above-described manufacturing methods of the
present invention, a preferable baking temperature in a reducing
atmosphere is in the range from 1,000.degree. C. to 1,100.degree. C.
The measured gas side electrode baked in this temperature range has
numerous micro pores which can enhance gas diffusing performance.
Furthermore, it becomes possible to improve the response of sensor output.
If the baking temperature exceeds 1,100.degree. C., alteration of a solid
electrolytic body will be caused.
Application of the present invention is not limited to a cup-shaped gas
sensing element (refer to FIGS. 1 and 2). Therefore, the present invention
can be applied to another type of gas sensing elements, such as a
multilayered planar gas sensing element consisting of a planar solid
electrolytic body, a planar measured gas side electrode or the like
stacked successively.
Furthermore, application of the present invention is not limited to an
oxygen concentration cell type element. Therefore, the present invention
can be applied to another type of gas sensing elements, such as a
limit-current type element, a lean sensor type element, and an air-fuel
ratio sensing element.
Furthermore, the gas sensor of the present invention can be used as a NOx
sensor, a HC sensor, or a CO sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will become more apparent from the following detailed
description, which is to be read in conjunction with the accompanying
drawings, in which:
FIG. 1 is a cross-sectional view showing an essential arrangement of a gas
sensing element in accordance with a preferred embodiment of the present
invention;
FIG. 2 is a cross-sectional view showing a gas sensor in accordance with
the preferred embodiment of the present invention;
FIG. 3 is a graph showing a relationship between X-ray diffraction angle
and diffraction strength with respect to each of sample 8 and a reference
sample in accordance with the preferred embodiment of the present
invention;
FIG. 4 is a graph showing a relationship between cumulative Si poisoning
time and change rate of control .lambda. with respect to each of sample 8
and the reference sample in accordance with the preferred embodiment of
the present invention; and
FIG. 5 is a graph showing a relationship between air-fuel ratio and voltage
of a conventional gas sensing element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be explained
hereinafter with reference to attached drawings. Identical parts are
denoted by the same reference numerals throughout drawings.
Gas Sensing Element
Hereinafter, a gas sensing element according to a preferred embodiment of
the present invention will be explained with reference to FIGS. 1, 2 and
5.
As shown in FIG. 1, the gas sensing element of the preferred embodiment
comprises a solid electrolytic body 10, a reference gas side electrode 12
provided on a surface of the solid electrolytic body 10 so as to be
exposed to a reference gas, and a measured gas side electrode 11 provided
on another surface of the solid electrolytic body 10 so as to be exposed
to a measured gas.
A crystal face strength ratio of the measured gas side electrode 11
according to X-ray diffraction is 1.3.ltoreq.{I(200)+I(220)}/I(111).
The gas sensing element 1 of this embodiment is incorporated in a gas
sensor 9 shown in FIG. 2 which is later described. The gas sensor 9 is
installed in an exhaust gas system of an automotive vehicle to control the
combustion of an internal combustion engine.
The gas sensing element 1 comprises a solid electrolytic body having an
oxygen ionic conductance. Two electrodes are provided on opposite surfaces
of the solid electrolytic body so as to cooperatively constitute an
electrochemical cell. A measured gas (exhaust gas) is introduced in the
vicinity of one electrode. A reference gas (air) is introduced in the
vicinity of the other electrode. An electric potential difference,
representing an oxygen concentration difference between the reference gas
and the measured gas, is produced between two electrodes. In this respect,
the gas sensing element 1 is a cell producing an electromotive force
representing an air-fuel ratio of the measured gas.
As shown in FIG. 1, the solid electrolytic body 10 has a cup-shaped
configuration defining therein a reference gas chamber 100 into which the
air is introduced. The reference gas side electrode 12 is provided on an
inner surface of the solid electrolytic body 10 so that the reference gas
side electrode 12 is exposed to the reference gas stored in the reference
gas chamber 100. The measured gas side electrode 11 is provided on an
outer surface of the solid electrolytic body 10.
The outer surface of the measured gas side electrode 11 is covered by a
first protective layer 13 which suppresses the diffusion of measured gas.
A second protective layer 14, serving as a trapping layer, covers the
outer surface of the first protective layer 13.
A rodlike ceramic heater 15 is disposed in the reference gas chamber 100. A
front end of heater 15 is brought into contact with the bottom of the
reference gas chamber 100.
The gas sensing element 1 has above-described characteristics of FIG. 5
showing the relationship between voltage and air/fuel ratio, according to
which a control .lambda. indicates a steep change point in the voltage. In
FIG. 5, a reference voltage is a criteria used for judging whether a fuel
injection amount should be increased or decreased in the combustion
control of an internal combustion engine. In general, the reference
voltage is set to 0.45 V.
Manufacturing Method (I)
The measured gas side electrode is manufactured in the following manner by
using a regular sintering process.
First, a solid electrolytic body is cleaned. Then, to form a Pt nucleus, a
solution containing 5 wt % chloroplatinic acid and a reducing solution
containing 10 wt % sodium tetrahydroborate are sprayed onto an electrode
forming portion where a measured gas side electrode is to be provided. The
Pt nucleus becomes a fine-grain nucleus (or a particulate core) of noble
metal.
The solid electrolytic body with the fine-grain nucleus thus formed is
cleaned in distilled water and then dried.
Next, the fine-grain nucleus is subjected to a two-hour reducing heat
treatment at a temperature range from 500.degree. C. to 800.degree. C. in
a reducing atmosphere consisting of 5 vol % H.sub.2 and 95 vol % N.sub.2
or in a reducing atmosphere consisting of 20 vol % H.sub.2 and 80 vol %
O.sub.2.
Thereafter, an electroless plating is applied to the Pt nucleus to form a
plating layer serving as a unbaked electrode film. Furthermore, the
unbaked electrode film is baked in a reducing atmosphere to obtain a
measured gas side electrode.
In this case, the heat treatment conditions for the electrode film are as
follows. The heat treatment was performed for one hour at the temperature
of 1,100.degree. C. in a reducing atmosphere consisting of 20 vol %
H.sub.2 and 80 vol % N.sub.2.
Gas Sensor Arrangement
The gas sensor 9 will be explained hereinafter.
FIG. 2 shows the gas sensor 9 incorporating a gas sensing element 1.
In addition to the gas sensing element 1, the gas sensor 9 has a housing 92
accommodating the gas sensing element 1.
The housing 92 has a barrel portion 93 with a flange 931 formed at the
center thereof. A measured gas side cover 94, to be placed in an exhaust
gas passage of an exhaust gas system, is connected to a distal end of the
barrel portion 93. An air side cover 95, to be exposed to the air, is
connected to a distal end of the barrel portion 93.
The measured gas side cover 94 consists of an inner cover 941 and an outer
cover 942 which are made of stainless steel and cooperatively constitute a
double-layer construction. These covers 941 and 942 have a plurality of
holes 943 and 944 opened to introduce the exhaust gas into the measured
gas side cover 94.
On the other hand, the air side cover 95 comprises a main cover 951
attached at one end to the barrel portion 93 and a sub cover 952
overlapping with the other end of the main cover 951. These covers 951 and
952 have holes for introducing air into the air cover 95.
The gas sensing element 1 is supported through an insulating member 932 by
the inside surface of the barrel portion 93. A metallic plate terminal 961
is connected to the reference gas side electrode 12 of the gas sensing
element 1 via a lead 911 and another metallic plate terminal 962 is
connected to the measured gas side electrode 11 of the gas sensing element
1 via a lead 912 in such a manner that the electrode leads 911 and 912 are
surrounded and clamped by the metallic plate terminals 961 and 962.
The plate terminals 961 and 962 are connected to output lead wires 971 and
972. More specifically, belt-like terminal pieces 963 and 964 protrude
from the plate terminals 961 and 962 toward contact pieces 965 and 966,
respectively.
The terminal pieces 963 and 964 are connected to lower ends 985 and 986 of
connectors 981 and 982. The other ends 983 and 984 of the connectors 981
and 982 are connected to the output lead wires 971 and 972.
The plate terminals 961 and 962 are respectively formed by deforming an
inverse T-shaped metallic plate into a cylindrical shape so as to hold the
lead 911 of reference gas side electrode 12 and the lead 912 of measured
gas side electrode 11.
An elastic spring force of the metallic plate gives an adequate pressing
force for clamping the electrode leads 911 and 912 by the plate terminals
961 and 962.
The lead wires 971 and 972 are respectively subjected to a tensile force
acting in an axial direction of the gas sensor 9. Thus, the lead wires 971
and 972 may pull the plate terminals 961 and 962 via the connectors 981
and 982, respectively. Thus, the plate terminals 961 and 962 may slide in
the axial direction.
A stopper 993, sandwiched between rubber bushes 991 and 992, is provided at
the proximal end of the gas sensor 9 to restrict the sliding of the plate
terminals 961 and 962. The stopper 993, also preventing the shifting of
connectors 981 and 982, is a resin-made member capable of insulating the
lead wires 971 and 972 from each other.
A wire 973 supplies electric power to the heater 15 of the gas sensing
element 1. The gas sensor 9 is fixed at the flange 931 to the wall of the
exhaust passage so that the measured gas side cover 94 protrudes in the
exhaust gas passage.
Hereinafter, functions and effects of this embodiment will be explained.
The measured gas side electrode is constituted by Pt which possesses
catalytic properties and has a face-centered cubic structure. In the
crystal lattice of Pt, crystal faces (100) and (110) have a lower surface
density of atoms and higher activity compared with other face (111)
dominant in this crystal lattice.
Accordingly, when the measured gas side electrode satisfies the
above-described condition that {I(200)+I(220)}/I(111) is equal to or
larger than 1.3, the crystal face having higher activity, i.e., an active
surface, is orientated on the surface of the measured gas side electrode.
Thus, even when the gas sensor is exposed to a high-temperature
environment for a long time, the gas sensor can produce a stable output.
The gas sensor becomes robust against Si poisoning and can maintain a
stable control .lambda. for a long time.
As described above, this embodiment can provide a gas sensing element which
is excellent in heat resistance as well as in Si poisoning durability.
Performance of a gas sensing element having a measured gas side electrode
of the present invention was evaluated through the following measurement.
A total of eight gas sensing elements ware prepared as test samples (refer
to Table 1). The measured gas side electrode of each test sample was
manufactured according the above-described first manufacturing method. As
apparent from Table 1, the manufactured gas sensing elements were
respectively differentiated in the conditions of a heat treatment applied
to the Pt nucleus serving as a fine-grain nucleus of noble metal. The
reducing heat treatment for each tested gas sensing element was conducted
for two hours.
For comparison, reference sample I was prepared as a gas sensing element
having not been subjected to the reducing heat treatment for the Pt
nucleus. The measured gas side electrode of reference sample I was
manufactured by directly applying the electroless plating to form a
plating layer of the electrode film and then baking the electrode film in
a reducing atmosphere. In this case, the baking processing was performed
for one hour at the temperature of 1,100.degree. C. in a reducing
atmosphere containing 20 vol % H.sub.2.
Manufacturing Method (II)
A second manufacturing method differs from the above-described first
manufacturing method in a step of applying laser irradiation processing to
a fine-grain nucleus of noble metal provided in an electrode forming
portion.
After applying the activation processing using chloroplatinic acid like the
above-described example 1, a ultraviolet laser beam with a diameter of 5
mm was irradiated entirely to the fine-grain nucleus for 30 minutes while
the laser output was changed in a range from 1 mW to 50 mW. The laser beam
irradiation was performed in an atmosphere consisting of 20 vol % H.sub.2
and 80 vol % O.sub.2. As a result of laser beam irradiation, the surficial
atoms of the fine-grain nucleus were reorientated so as to form the faces
(100) and (110).
Like the first manufacturing method, an electroless plating was applied to
form a plating layer of a unbaked electrode film. The electrode film was
baked in a reducing atmosphere to form a measured gas side electrode.
In this case, the baking processing was performed for one hour at the
temperature of 1,100.degree. C. in a reducing atmosphere consisting of 20
vol % H.sub.2 and 80 vol % N.sub.2.
Reference sample II was prepared as a gas sensing element fabricated from a
fine-grain nucleus having not been subjected to the laser irradiation.
Manufacturing Method (III)
A third manufacturing method differs from the above-described first
manufacturing method by steps of preparing a paste of a fine-grain nucleus
applied the crystal face orientation processing beforehand and then
forming a unbaked electrode film resulting from this paste.
First, chloroplatinic acid powder was subjected to a heat decomposition
within a temperature range from 600.degree. C. to 1,000.degree. C. in a
reducing atmosphere containing 20 vol % H.sub.2, thereby obtaining a
fine-grain nucleus of platinum having a grain size of several hundreds
.ANG. with faces (100) and (110) orientated on the surface thereof.
Next, a paste was formed by uniformly kneading the above-described platinum
fine-grain nucleus (70 wt %) with 5 wt % binder made of PVB (polyvinyl
butyral), 23 wt % terpineol solvent, and 2 wt % glass frit.
The paste was coated on an electrode forming portion by using a
conventionally known technique such as a screen printing or a roll
printing. After the paste having been applied to the electrode forming
portion was dried, binder removing processing was performed in the air for
one hour at the temperature of 600.degree. C., thereby obtaining an
electrode film.
Thereafter, like the first manufacturing method, the electrode film was
baked in a reducing atmosphere to form a measured gas side electrode.
In this case, the heat treatment of the electrode film was performed for
two hours at the temperature of 1,100.degree. C. in a reducing atmosphere
consisting of 20 vol % H.sub.2 and 80 vol % N.sub.2.
Reference sample III was prepared as a gas sensing element fabricated
without applying the reducing heat treatment to the Pt nucleus.
Measurement of Crystal Face Strength Ratio
A measurement of crystal face strength ratio was performed on respective
measured gas side electrodes of the gas sen
