CA1151729A - Oxygen sensor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An oxygen sensor for detecting an oxygen concentration in a gas under test such as exhaust gas from an internal combustion engine of an automobile, The oxygen sensor comprises an oxygen concentration sensing ele-ment made of an oxygen ion conductive metal oxide which produces an electro-motive force in accordance with a difference between an oxygen concentration in the gas under test and an oxygen concentration in a reference gas, a porous layer of refractory metal oxide formed on a surface of the oxygen concentration sensing element which is to be exposed to the gas under test, and an electrode formed thereon. The peel-off of the electrode is prevented and a high-response oxygen sensor is provided, These sensors find use in fuel control systems in which the air : fuel ratio in gas for supply to an internal combus-tion engine is controlled at a constant value, These sensors exhibit a high response and better durability than the known sensors,

Description

l The present invention relates to an oxygen sensor for sensing an oxygen concentration in gas components of a gas under test such as exhaust gas from an internal combustion engine oP an automobile, and more particularly to a high-response oxygen sensor which can prevent the peel-off of an electrode on the side of the sensor which is to be exposed to the gas under test.
A prior art oxygen sensor includes an oxygen concentration sensing element made of an oxygen ion conductive metal oxide such as ZrOz-CaO, and thin film platinum (Pt) electrodes are formed on the surfaces of the oxygen concentration sensing element which are to be exposed to a reference gas and the gas under test, respectlvely, by chemical plating, vapor deposition or the like.
In the prior art sensor, the oxygen concentra-tion sensing element is highly sintered to form a fine structure because an electromotive force will not be produced iP the gas transmits through the oxygen concentration sensing element as the oxygen concentration sensing element is to sense a differential oxygen concentration between the gas under test and the refe-rence gas. As the oxygen concentration sensing element is highly sintered, the adhesion of the electrodes to the sensing element is so weak that when they are left exposed to the gas under test for a long tiMe they are peeled off from the surface of the oxygen concentration sensing element. A difference between thermal expan-sions of the electrodes and the sensing element also ~151~9 contributes to the peel-off of the electrode.
In order to resolve the above problem, it has been proposed to sand-blast the surface of the oxygen concentration sensing element, which is exposed to the gas under test, to make the surface rough and form fine projec-tions and recesses (namely, to give surface porosity) in order to enhance the adhesion of the electrode and improve the anti-peel-off property of the elec-trode. HoweverJ although the sand-blasted oxygen concentration sensing ele-ment has an improved anti-peel-off property, fine cracks are formed on the surface of the oxygen concentration sensor so that a mechanical strength there-of is lowered and the oxygen concentration sensor may be destroyed when it is ; used in a vibrating environment for a long time. Thus, it has a problem of low durability In order to improve the response and the precision of the oxygenconcentration sensing element, it has been proposed, as disclosed in United States Patent 3,935,089* to form a porous inorganic material on an outer surface of tho eloctrode to carry thereon a catalytic material to promote equilibrium of oxy~en partial pressure so that an electromotive force changes abruptly near a stoichiometric air-fuel ratio. Since this type of oxygen sensor is not prac-tical without the catalytic material carried on the outer surface of the elec-trode, the cost of the sensor is increased when a noble metal such as platinumis used as the catalytic material.
It is an object of the present invention to *issued January 27, 1976, assigned to Nissan Motor Co. Ltd.

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~S31~729 provide an oxygen sensor having an oxygen concentration sensing element in which a porous film made of a refractory metal oxide is formed on a surface of the oxygen concentration sensing element which is to be exposed to a gas undertest and an electrode is formed on the porous film.
It is another object of the present invention to provide an oxygen sensor which prevents the peel-off of the electrode, has a - high response and is inexpensive.
According to the first aspect of the invention there is provided an oxygen sensor comprising:
an oxygen concentration sensing element made of an oxygen ; ion conductive metal oxide which produces an electromotive force in accordance with a difference between an oxygen concentration in gas components of gas under test and an oxygen concentration in a refer-ence gas;
a first porous film made of a refractory metal oxide formed on a surface of said oxygen concentration sensing element which is to be exposed to the gas under test; and an electrode consisting of a metal having a catalytic action to oxygen ions formed on said first porous film.
According to the second aspect of the invention there is provided an oxygen sensor for detecting an oxygen concentration in an exhaust gas from an internal combustion engine of an automobile, comprising: an oxygen concentration sensing element made of an oxygen ion conductive metal oxide mixture selected from a group consisting of ZrO2-Y2O3, ZrO2-CaO and ZrO2-MgO, which produces an i electromotive force in accordance with a differencebetween an oxygen concentration in the exhaust gas and an oxygen concentration in a .
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reference gas; a first porous film made of a refractory metal oxide formed on a surfaee of said oxygen concentration sensing element whieh is to be exposed to the exhaust gas said refractory metal oxide being an oxygen ion conducting metal of the same composition as said oxygen concentration sensing element; a first eleetrode eonsisting of porous layer of metal having a eatalytie action, said first eleetrode being formed on said first porous film in a condition that component grains of said first electrode penetrate into said first porous film; a second electrode formed on an opposite surfaee of said oxygen eoneentration sensing element.
According to the third aspect of the invention there is provided an oxygen sensor for detecting an oxygen concentration in an exhaust gas from an internal combustion engine of an automobile, eomprising: an oxygen eoncentration sensing element made of an oxygen ion eonduc'ive metal oxide mixture selected from a group eonsisting of ZrO2-Y2O3, ZrO2-CaO and ZrO2-MgO, which produces an eleetromotive foree in aeeordanee with a differenee between an oxygen eoneentration in the exhaust gas and an oxygen eoneentration in a referenee gas; a first porous film made of a refractory metal oxide formed on a surface of said oxygen concentration sensing element which is to be exposed to the exhaust gas said refractory metal oxide being an oxygen ion conducting metal and selected from the group eonsisting of ZrO2-Y2O3, ZrO2-CaO and ZrO2-MgO, said first porous film having a surface chemieally etehed by a strong aeid and upon whieh said first electrode is formed; a first electrode consisting of a porous layer of metal having a catalytie action, said Pirst electrode being formed on said first porous film in a condition that component grains of said first electrode penetrate - 3a -~ii17~29 into said first porous film; a second porous film formed on saidfirst electrode for protecting said first electrode from the exhaust gas; and a second electrode formed on an opposite surface of said oxygen concentration sensing element.
The oxygen concentration sensing element and the porous film used in the oxygen sensor of the present invention are both metal oxides and hence adaptable to each other, so that the film strongly adheres to the surface of the oxygen concentration sensing element. Furthermore, since the film is porous and hence has a large surface area, the electrode strongly adheres to the film.
Therefore, the adhesion of the electrode to the oxygen concentra-tion sensing element can be enhanced without surface treatment such as sand blasting. As a result, the problem of the destruction of the oxygen concentration sensing element due to the sand blasting is avoided and a high durability oxygen sensor can be provided.
In the present invention, while any refractory metal oxide may be used as a material for the porous film, a material of the same composition as the oxygen concentration sensing element or of ~imilar composition to the latter is preferable. When the porous film has such a composition, the oxygen ion conduction is not impeded and a stable characteristic of the oxygen - 3b -1 concentration sensing element is obtained. It should be understood, however, that in attaining the object of the present invention of enhancing the adhesion of the electrode any refractory metal oxide may be used.
These and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment of the invention, when taken is conjunction with the accompanying drawings, in which;
Fig. 1 is a sectional view illustrating one embodiment of an oxygen sensor of the present lnvention;
Fig. 2 is a system diagram showing a section A in Fig. 1 in an enlarged scale;
Figs. 3 and 4 show response characteristics of the ox~gen sensor in accordance with the embodiment of the present invention and a prior art ox~gen sensor;
and Fig. 5 shows responses to durability time of the ox~gen sensor in accordance with the embodiment of the present invention and the prior art oxygen sensor.
Referring to Fig. 1, numeral 1 denotes an ox~gen concentration sensing element made of an ox~gen ion conductive metal oxide, which is a finely sintered body of 70 - 95 mole % of metal oxide such as ZrO2, ThO2 or CeO2 to which 30 - 5 mole % of dlvalent or trivalent metal oxide ~1as dissolved by solid solution.
In the illustrated embodiment, 90 mole ~ of ZrO2 and 10 mole % of Y2O3 are mixed, and the mix~ure is ~151729 1 ground and calcined and formed into a desired cup-shape, which is then fired at approximately 1600 - 1750C to form a finely sintered body. A porous film 1' is formed on an outer circumference of the oxygen concentration sensing element 1. It is formed by grinding the calcined mixture of 90 mole % of ZrO2 and 10 mole % of Y203 (grain size of 1 - 20 ~), adding small amount of water to make the mixture muddy, spray-ing the muddy mixture on the outer circumference of the oxygen concentration sensor 1 or painting it to the thickness of 20 - 200 ~ and firing the oxygen concent-ration sensor 1 at 1600 - 1750C. Numeral 2 denotes a porous layer of a first electrode which is formed on a surface of the film 1' by depositing thereon platlnum having a catalytlc action by chemical plating, vapor deposltion or paste application, and numeral 3 denotes a second electrode formed by depositlng platlmum ha~lng a catalytic action on an inner circumference of the oxygen concentration sensing element 1 by chemlcal plating or paste application. A porous coating 4 of the thickness of 50 - 100 ~ is formed on the surfaces of the first electrode 2 in order to protect the first electrode 2 from the exhaust gas and stabllize an output of the detector. The coating 4 may comprise an oxide or composite oxide such as A1203, Al2o3 - sio2, r~go - A1203 or ZrO2. Numer~l 5 denotes a housing which ls used to directl~ scre-,l the detector to an exhaust pipe 10 and has a threaded portion 5a at the bottom thereof. The oxygen concentration sensing element 1 and the housing 5 are secured to each other llS17Z9 1 by pressing an 0-ring 7 with a conductive metal material 6 which is interposed therebetween. Numeral 8 denotes a metal stem pressed into the bore of the sensing element l with a conductive material 6' interposed between the ox~Jgen concentration sensing element l and the stem 80 The stem 8 is formed with a bore 8a through which the inner circumference of the ox~gen concentration sensing element 1 is exposed to the atmosphere as a reference gas. The housing 5 is electricall~ connected to the first electrode 2 through the conductive material 6 so that the housing 5 con-stitutes one of the electrodes from which an output is taken, while the stem 8 is electrically connected to the second electrode 3 through the conductive material 6' so that the stem 8 constitutes the other electrode. Numeral 9 denotes a protecti.ve metal tube having a number of pores 9a, which is provided to relax the direct contact of the outer circumfernce of the ox~gen concentration sensing element 1 to the

2~ exhaust gas. Numeral 11 denotes a conductive ring.
In Fig. 2, nurneral la denotes component grains of the ox~gen concentration sensing element 1 and numeral la' denotes component grains of the film 1'. ~Tumeral 2a denotes component grains of the electrode 2 and Z5 numeral 4a denotes component grains of the coating 4.
The operation of the structure described above is now explained. The ox~gen sensor is mounted on the exhaust pipe 10 b~ the housing 5 so that the ox~gen concentration sensing element 1 is exposed to the exhaust gas. As is well known, the exhaust gas llS17Z9 1 comprises component gases such as 2~ CO and HC, and the concentrations of those component gases change with an air-fuel ratio before combustion of the air-fuel mixture.
The oxygen concentration sensing element 1 produces an electromotive force in accordance with a difference bet-ween an oxygen concentration in the exhaust gas and an oxygen concentration in the atmosphere which acts as a reference gas. The electromotive force is high (0.9 V) on the rich side of the air-fuel ratio and low (0.1 V) on the lean side of the air-fuel ratio, and abruptly changes at a stoichiometric air-fuel ratio. N~Ox component is less on the rich side of the air-fuel ratio while HC and C0 components are less on the lean side of the air-fuel ratio, and it is near the stoichiometric ratio that the amount of those three components is minimum.
Accordlngl~ by managing the electromotive force of the detector at the stoichiometric air-f'uel ratio to control the ,air-fuel ratio of the air-fuel mixture, the exhaust gas can be purifled in a very efficient way.
In the present embodiment, since the film 1' formed on the surface of the oxygen concentration sensing element 1 has the same composition as t~e oxygen concentration sensing element and thereby thermal expansions thereof are equal to each otner, the~ are adaptable to each other and the adhesion of the film 1' to the oxygen concentration sensing element,l is very strong. Accordingly, the film 1' is not peeled off and the oxygen ion conductivity is not affected. Since the film 1' is porous, the contact area of the electrode 2 to the film 1' increases and the component grains of 1151~29 1 the electrode 2 deeply penetrate into the film l'.
Accordingly, the adhesion of the electrode 2 to the film 1' is very strong and the electrode 2 is not peeled off from the film l' even after the long term usage, and hence the electrode 2 is not peeled off from the oxygen concentration sensing element l.
Furthermore, when the electrode 2 is made of a metal having a catalytic action such as platinum, since the platinum electrode is carried by the porous film l' to provide a large catalytic active area, it is not always necessary to carry another metal having a catalytic action on the outer surface of the electrode 2. The inventors of the present invention have found the following from an experiment. Figs. 3 and 4 show the comparative responses of the sensor of the present invention (Sample A: with the film 1' of ,'~e thickness of 50 ~) and a prior art sensor (Sample B:
with an electrode directly formed on the surface of the sensing element). Figs. 3 and 4 show the response times of the samples A and B in an engine havlng six cylinders and a capacity of 2000 cc, at a re~lolution speed of 2000 rpm and at an exhaust gas temperature of 450C. Fig. 3 shows the response times measured when an air excessive factor ~ of air-fuel mixture to be fed to the engine was changed from 0.9 to l.l (air-fuel ratio: rich to lear) so that an OUtpllt voltage of the sensor changed frcm o.6 V to 0.3 V, and F'ig. 4 shows the response time measured when the air excessi~Je factor was changed from l.l to 0.9 (air-fuel ratio:
lean to rich) so that the output voltage of the sensor _ ~ _ ~1517Z9 l changed from 0.3 V to o.6 V. In Figs. 3 and 4, the solid lines show the output voltage characteristics of the sensor of the present invention (Sample A) and the broken lines show the output voltage characteristics of the prior art sensor (Sample B). It is apparent from the above experiment that the response of the sensor of the present invention is much superior to that of the prior art sensor.
Another experiment was carried out to determine the durability and the anti-peel-off property of the sensor of the present invention. The sensor of the present invention shown in Fig. 1 (Sample A:
with the film 1' having the thickness of 50 ~) and the prior art sensor (Sample B) were placed in an electric oven at 1000C for 20 minutes, and they were then cooled to a room temperature and then again placed in the electric oven at 1000C for 20 minutes.
The above c~cle was repeated 200 times. In the sensor shown in Fig. l, the electrode 2 was not peeled off at all, while in the prior art sensor in which the electrode was formed directl~ on the surface of the ox~gen concentration sensing element, the ele^trode was partiall~ peeled off. Further, a bending strength was mea~ured for the ox~gen concentration sensing element having the surface sand blasted in a conventional manner. The result was that the bending strength thereof was lower than t.-,at of the oxygen concentration sensing element of the present embodiment having the surface not sand blasted, by approximately 50 %.
A further experiment was carried out to _ 9 _ ~ ~S1729 1 determine the durability and the response of the sensor of the present invention. The sensor of the present invention (Sample A) and the prior art sensor (Sample B) T~ere tested for the durability under the following operation conditions and the response times of the Samples A and B under the respective durability times when ~ was changed from 0.9 to 1.1 were measured.
The results are shown in Fig. 5.
Operation conditions:
. engine; 2000 cc, six cylinders.
. exhaust gas temperature; 850C
. air-fuel ratio; 14 It is seen from Fig. 5 that in the Sample B
the response time becomes longer (deterioration is meant) when the durability test is continued over 500 hours, while in the Sample A the response time does not substantially change even after 1000 hour durability.test.
Furthermore, in the present embodiment, since the porous coating 4 is formed on the surface of the electrode 2, lead, oil and phosphorus in the exhaust gas deposit on the coating 4 but do not deposit directl~ on the electrode 2, and hence the catal~tic actlon of the electrode 2 is not deteriorated.
In essence, the coating 4 functions as a filter.
The present invention is not limited to the illustrated embodiment but various modificatlons as shown below ma~ be made:
(1) The firing process of the material of the film 1' may be omitted b~ depositing the material of -- 10 _ ~51729 1 the film 1' on the surface of the cup-shaped mold which is to form the oxygen concentration sensing element 1 before the mold is fired and then firing the mold.
- (2) Although the material of the film 1' is the same as the material of the oxygen concentration sens-ing element 1 (solid solution composition of ZrO2 and Y2O3), it may be an oxygen ion conductive metal oxide i other than ZrO2 - Y2O3, such as ZrO2 - CaO or ZrO2 -: MgO, or even a metal oxide other than the oxygen ion conductive metal oxide, such as A12O3 or MgO - A12O3.

(3) The material of the film 1' may be deposited on the surface of the oxygen concentration sensing element 1 by plasma injection-welding and then firing the sensing element at 1400 - 1600C in order to further enhance the adhesion between the oxygen concentratlon sensing element 1 and the film 1'.

(4) The surface of the film 1' may be chemically etched b~ strong acid such as fluoric acid before the electrode 2 is deposited on the surface of the film 1' in order to further enhance the adhesion between the film 1' and the electrode 2.

(5) While the conductive material of the electrodes 2 and 3 is platinum in the illustrated embodiment, it may be other material having a catalytic action such as Pd, Rh, Au, Ru or Ag, or an allo~ thereof, or different metals ma-~ be used for the electrodes 2 and 3, such as Pd for the electrode Z, Pt for the electrode 3 and Pd for the thick film.

(6) ~Ihile the oxygen concentration sensing element 1 ~151729 1 is of cup-shape having one end opened and the other end closed in the illustrated embodiment, it may be of plate shape or cylinder shape.
Furthermore, the present invention is not limited to means for sensing the oxygen concentration in the exhaust gas from the internal combustion engine to detect the air-fuel ratio of the air-fuel mixture to be fed to the internal combustion engine as shown in the illustrated embodiment, but the present invention may be used as means for sensing an oxygen concentration in a combustion product exhausted from - a combustion mechanism such as a blast furnace or boiler to detect an air-fuel ratio of air-fuel mixture to be supplied to the combustion mechanism (for example, for lmprovlng a thermal efficiency of the cornbustion mechanlsm).
An experiment by the inventors has proved that when the oxygen concentratlon sensing element 1 was made of an oxygen lon conductive metal oxide conslsting of 90 - 92 mole % of ZrO2 and 10 - 8 mole %
of Y203, the conductivity is so lncreased that the conduction of the oxygen ion was carried out even at a low temperature.
As described hereinabove, according to the present lnvention, slnce the film of the refractory porous metal oxide is formed on the surface of the oxygen concentration sensing element which is to be exposed to the gas under test and the electrode is - formed on the surface of the film, both the ox~Jgen concentration sensing element and the film are made of 1 the metal oxide and they are compatible with each other.
Accordingly, the film is strongly adhered to the oxygen concentration sensing element. Further, since the film is porous, it has a large contact area with the electrode and a portion of the electrode penetrates into the pores of the film. Therefore, the electrode is strongly adhered to the film and the adhesion of the oxygen concentration sensing element to the electrode is very strong without requiring the sand blasting pr^cess. Accordingly, the problem of decrease of the strength of the oxygen concentration sensing element due to sand blasting, which was encounted in the prior art sensor, is not raised.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An oxygen sensor comprising:
an oxygen concentration sensing element made of an oxygen ion conductive metal oxide which produces an electromotive force in accordance with a difference between an oxygen concentration in gas components of gas under test and an oxygen concentration in a refer-ence gas;
a first porous film made of a refractory metal oxide formed on a surface of said oxygen concentration sensing element which is to be exposed to the gas under test; and an electrode consisting of a metal having a catalytic action to oxygen ions formed on said first porous film.

2. An oxygen sensor according to claim 1 further comprising a second porous film formed on said electrode.

3. An oxygen sensor according to claim 1 wherein said oxygen concentration sensing element and said first porous film have the same composition.

4. An oxygen sensor according to claim 1 wherein said elec-trode is made of a metal having a catalytic action selected from a group consisting of Pt, Rh, Au, Ru and Ag or an alloy thereof.

5. An oxygen sensor according to claim 1 wherein said oxygen concentration sensing element consists of 90 - 92 mole % of ZrO2 and 10 - 8 mole % of Y2O3.

6. An oxygen sensor according to claim 1 wherein said elec-trode is made of a metal having a catalytic action selected from the group consisting of Pt, An and Ag.

7. An oxygen sensor for detecting an oxygen concentration in an exhaust gas from an internal combustion engine of an auto-mobile, comprising: an oxygen concentration sensing element made of an oxygen ion conductive metal oxide mixture selected from a group consisting of ZrO2-Y2O3, ZrO2-CaO and ZrO2-MgO, which pro-duces an electromotive force in accordance with a difference between an oxygen concentration in the exhaust gas and an oxygen concentration in a reference gas; a first porous film made of a refractory metal oxide formed on a surface of said oxygen concen-tration sensing element which is to be exposed to the exhaust gas said refractory metal oxide being an oxygen ion conducting metal of the same composition as said oxygen concentration sensing ele-ment; a first electrode consisting of porous layer of metal having a catalytic action, said first electrode being formed on said first porous film in a condition that component grains of said first electrode penetrate into said first porous film; a second electrode formed on an opposite surface of said oxygen concentra-tion sensing element.

8. An oxygen sensor for detecting an oxygen concentration in an exhaust gas from an internal combustion engine of an automobile, comprising: an oxygen concentration sensing element made of an oxygen ion conductive metal oxide mixture selected from a group consisting of ZrO2-Y2O3, ZrO2-CaO and ZrO2-MgO which produces an electromotive force in accordance with a difference between an oxygen concentration in the exhaust gas and an oxygen concentration in a reference gas;
a first porous film made of a refractory metal oxide formed on a surface of said oxygen concentration sensing element which is to be exposed to the exhaust gas said refractory metal oxide being an oxygen ion conducting metal and selected from the group consisting of ZrO2-Y2O3, ZrO2-CaO and ZrO2-MgO, said first por-ous film having a surface chemically etched by a strong acid and upon which said first electrode is formed; a first elec-rode consisting of a porous layer of metal having a catalytic action, said first electrode being formed on said first porous film in a condition that component grains of said first electrode penetrate into said first porous film; a second porous film formed on said first electrode for protecting said first electrode from the exhaust gas; and a second electrode formed on an opposite surface of said oxygen concentration sensing element.

9. An oxygen sensor according to claim 1, 7 or 8 wherein said first porous film is formed on the surface of said oxy-yen concentration sensing element through deposition of a material of said first porous film, using plasma injection welding.

10. An oxygen sensor for detecting an oxygen concentration in an exhaust gas from an internal combustion engine of an automo-bile, comprising: an oxygen concentration sensing element made of an oxygen ion conductive metal oxide mixture of ZrO2-CaO which pro-duces an electromotive force in accordance with a difference between an oxygen concentration in the exhaust gas and an oxygen concentra-tion in a reference gas; a first porous film made of a refractory metal oxide formed on a surface of said oxygen concentration sensing element which is to be exposed to the exhaust gas said refractory metal oxide being an oxygen ion conducting metal of the same compo-sition as said oxygen concentration sensing element; a first elec-trode consisting of a porous layer of metal having a catalytic action, said first electrode being formed on said first porous film in a condition that component grains of said first electrode pene-trate into said first porous film; a second electrode formed on an opposite surface of said oxygen concentration sensing element.

11. An oxygen sensor according to claim 1 or 10 wherein said first porous film is formed on the surface of said oxygen concentra-tion sensing element through deposition of a material of said first porous film, using plasma injection welding.