GB2050628A

GB2050628A - Flat Type Thin Film Oxygen Sensor

Info

Publication number: GB2050628A
Application number: GB8017291A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1979-05-25
Filing date: 1980-05-27
Publication date: 1981-01-07
Also published as: GB2050628B; JPS55156858A; DE3019824C2; FR2457489B1; DE3019824A1; FR2457489A1

Abstract

A flat type thin film oxygen sensor comprises a layer (28) of an oxygen ion conductive solid electrolyte, a porous and electronically conductive reference electrode layer (26) formed on one side of the electrolyte layer, a porous and electronically conductive measurement electrode layer (30) formed on the other side of the electrolyte layer, and a partition layer (24) of an electrochemically inactive material formed on the outer side of the reference electrode layer. The reference and measurement electrode layers are constructed of cermet materials. At least one of the solid electrolyte layer 28 and partition layer 24 is permeable to fluids and the reference layer communicates with the external atmosphere only through the permeable layer(s). Either the solid electrolyte layer of the partition layer functions as the structural base layer of the sensor. The partition layer may incorporate a heater. <IMAGE>

Description

SPECIFICATION Flat Type Thin Film Oxygen Sensor The present invention relates in general to an oxygen sensor for measuring the oxygen concentration in a fluid, the sensor being of the type having a layer of an oxygen ion conductive solid electrolyte. More specifically, the present invention is concerned with a so-called flat thin film oxygen sensor which is suitable for measuring the oxygen concentration in an engine exhaust gas issued from the engine or a charged gas to be fed to the engine.

In recent years, a flat thin film oxygen sensor using a flat thin film solid electrolyte has been used in oxygen sensors as a substitute for a hitherto used tubular oxygen sensor using a tubular solid electrolyte. Although the flat thin film oxygen sensors now in existence exhibit good performances in EMF (electromotive force) characteristic and responsiveness, some of them have a drawback in durability, as will become clear as the description proceeds. It often happens that mutually attached element layers forming the thin film oxygen sensor come off or peel off from one another during the practical usage thereof.

It is therefore an essential object of the present invention to provide an improved thin film oxygen sensor.

According to the present invention, there is provided an oxygen sensor comprising: a layer of an oxygen ion conductive solid electrolyte; a porous and electronically conductive reference electrode layer formed on and in intimate contact with one side of the electrolyte layer; a porous and electronically conductive measurement electrode layer formed on and in intimate contact with the other side of the electrolyte layer; and a partition layer of an electrochemically inactive material formed on and in intimate contact with the outer side of the reference electrode layer, at least one of the solid electrolyte layer and the partition layer having a porous structure permeable to fluids and being formed such that the reference electrode layer communicates with an exterior atmosphere only through the porous structure, one of the solid electrolyte layer and the partition layer being so constructed and formed to serve as a structurally basic member of the oxygen sensor, wherein cermet is used as the material of the reference and measurement electrode layers.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, wherein.

Fig. 1 is a cross-section of a conventionally used flat thin film oxygen sensor; Figs. 2 to 5 are views similar to Fig. 1, but show respectively four embodiments of the present invention; Figs. 6a to 69 are views depicting in order the production process of an embodiment of an oxygen sensor according to the present invention; Fig. 7 is a cross-section view of a holder which is used for holding the oxygen sensor; Figs. 8a to 8c are views depicting in order the production process of another embodiment of an oxygen sensor according to the present invention; Figs. 9 and 10 are graphs both showing the performance of an embodiment of an oxygen sensor according to the invention, the performance being represented by relationship between time elapsed, variation in gas atmosphere and electromotive force generated; and Fig. 11 is a graph also showing the performance of an embodiment of an oxygen sensor according to the invention, the performance being represented by relationship between gas temperature and electromotive force.

Prior to describing in detail embodiments of an oxygen sensor according to the invention, outlined explanation of one of conventionally used flat thin film oxygen sensors using a flat thin film oxygen ion conductive solid electrolyte will be made with reference to Fig. 1 in order to clarify the invention.

Referring to Fig. 1, the side cross-section of the conventional oxygen sensor is shown as being designated by reference numeral 10. The sensor 10 generally comprises a base plate 12 constructed of ceramics, a reference electrode layer 14 deposited on the base plate 12, a layer 1 6 of an oxygen ion conductive solid electrolyte, and a measurement electrode layer 1 8 deposited on the solid electrolyte layer 1 6. Leads 20 are connected via a potentiometer 22 to the reference and measurement electrode layers 14 and 1 8 to cause the potentiometer 22 to measure the electromotive force (EMF) generated between the electrodes 14 and 1 8.Alumina (Al2O3) is used as the material of the base plate 1 2, platinum (Pt) is used for the electrode layers 14 and 18, and yttria stabilized zirconia (Y2O3-ZrO2) or calcia stabilized zirconia (CaO-ZrO2) is used as the material of the solid electrolyte layer 1 6.

In this type of oxygen sensor 10, the following three regions as well as the region defined between the base plate 1 2 and the solid electrolyte 1 8 form independently a so-called metal-ceramic boundary surface or metal-ceramic interface.

(1) The region defined between the base plate 12 and the reference electrode layer 14.

(2) The region defined between the reference electrode layer 14 and the solid electrolyte layer 16.

(3) The region defined between the solid electrolyte layer 1 6 and the measurement electrode layer 18.

Observation of these interfaces by using a scanning electron microscope has revealed that the bond occuring at those regions is not made by chemical adhesion, but physical adhesion or mechanical adhesion, which means that the bond at those regions is not strongly made. In fact, it often happens that the attached layers 14, 1 6 and 1 8 on the base plate 12 come off or peel off from one another during the practical usage of the sensor because of different expansion rates of these layers. This unwanted phenomenon becomes more critical when such a sensor is installed in an automotive engine exhaust system. In using the sensor in the exhaust system for measurement of the oxygen concentration in the exhaust gas from the engine, the sensor is compelled to run enduring not only high temperature of the exhaust gas but also high speed flow of the same.Particularly, the measurement electrode layer 18 tends to come off from the solid electrolyte layer 1 6.

Throughout the following description, like parts are designated by the same reference numerals.

Referring to Fig. 2 of the drawings, there is shown an oxygen sensor according to the first embodiment of the present invention, which is designated by reference 1 OA. The oxygen sensor 1 OA comprises a partition layer 24 which is so designed as to serve as a structurally basic member of the sensor 1 OA. A reference electrode layer 26 of electron conductive cermet is deposited on the surface of the partition layer 24, and a layer 28 of an oxygen ion conductive solid electrolyte is deposited on the surface of the reference electrode layer 26. As shown, the inner side of the reference electrode layer 26 is entirely and intimately covered with the electrolyte layer 28. A measurement electrode layer 30 of electron conductive cermet is deposited on the outer side of the solid electrolyte layer 28.Leads 20 are connected via a potentiometer 22 to the reference and measurement electrode layers 26 and 30.

Now, it should be noted that the electrode layers 26 and 30 and the solid electrolyte layer 28 have a microscopically porous structure, i.e. a gas permeable structure, while the partition layer 24 has a structure impervious to gas.

If desired, a DC power source 38 (preferably a constant current DC power source) may be connected in parallel with the potentiometer 22 in a manner as shown by broken lines to enforcedly cause a DC current to flow from one side of the solid electrolyte layer 28 to the other side. As will be understood from the description provided hereinafter, addition of such DC power source 28 stabilizes the oxygen partial pressure at the interface between the reference electrode layer 26 and the solid electrolyte layer 28, permitting generation of stable electromotive force (EMF).

Referring to Fig. 3, there is shown the second embodiment of the invention. The oxygen sensor 1 OB of this embodiment comprises substantially the same parts as in the case of the first embodiment 1 OA except that in this second embodiment, a heating member 32 is embedded in the partition layer 32. By the provision of the heating member 32, the oxygen ion conductivity of the solid electrolyte layer 28 at low temperature is considerably improved. For achieving effective heating of the solid electrolyte layer 28, the heating member 32 should be located as close as possible to the electrolyte layer 28.

Referring to Fig. 4, there is shown the third embodiment of the present invention. The oxygen sensor 1 OC of this embodiment is an example in which the solid electrolyte layer 36 is designed to serve as a structurally basic member of the sensor. The sensor 1 0C comprises a partition layer 34 having a gas permeable structure, a reference electrode layer 28 of cermet deposited on the partition layer 34, a solid electrolyte layer 36 deposited on the reference electrode layer 26 and a measurement electrode layer 30 of cermet deposited on the electrolyte layer 36 as shown, the outer side of the reference electrode layer 26 is covered with the partition layer 34. The solid electrolyte layer 36 has a structure impervious to gas.

Referring to Fig. 5, there is shown the fourth embodiment of the present invention. The oxygen sensor 1 0D of this embodiment comprises substantially the same parts as in the case of the first embodiment (Fig. 2) except that in the fourth embodiment a protective layer 38 having a porous or gas permeable structure is used for covering both the measurement electrode layer 30 and side surfaces of the solid electrolyte layer 28 entirely and intimately, as shown. With this construction, the measurement electrode layer 30 is protected from direct exposure to an exterior atmosphere in which the sensor 1 0D is arranged, thereby prolonging the life-time of the measurement electrode layer 30.

As will become clear hereinafter, several experiments have revealed that the oxygen sensors having the abovementioned construction are excellent in durability as compared with the conventional oxygen sensors such as one 10 shown in Fig. 1. Further, it has been revealed that addition of proper amounts of components of the electrolyte layer 28 (Figs. 2,3 8 5) or 36 (Fig. 4) and of components of the partition layer 24 (Figs. 2, 3 Er 5) or 34 (Fig. 4) to the reference electrode layer 26 increases bonding forces appearing at the interfaces between the electrolyte layer 28 or 36 and the reference electrode layer 26 and the partition layer 24 or 34, and that addition of proper amounts of components of the electrolyte layer 28 or 36 to the measurement electrode layer 30 increases bonding force appearing at the interface therebetween.

Further, as will be understood from hereinafter prepared description, using an electron conductive cermet as the material of the reference and measurement electrode layers 26 and 30 causes, upon deposition of the layers at high temperature, occurrence of diffusion of atoms or ions at the respective interfaces defined by the partition layer 24 or 34, the reference electrode layer 26, the solid electrolyte layer 28 or 36 and the measurement electrode layer 30, which means formation of chemical or strong adhesion in each interface.

Furthermore, it has been revealed by the experiments that if the partition layer, the reference electrode layer of electron conductive cermet, the electrolyte layer and the measurement electrode layer of electron conductive cermet are heaped on one another in green conditions thereof and then the thus temporarily bonded layers are baked or sintered, the oxygen sensor thus produced exhibits excellent durability against the layer peeling-off phenomenon.

The following description is directed to the principle of generation of pulsating electromotive force. For convenience, the description will be made with reference to the first embodiment of Fig. 2 of the drawings.

When the oxygen sensor 1 OA is disposed in an oxygen-containing gas atmosphere which temporally provides an oxygen-rich gas having higher oxygen partial pressure, the gas comes into contact with the reference electrode layer 26 through the openings of the measurement electrode layer 30 and the solid electrolyte layer 28 thereby causing the interface between the reference electrode layer 26 and the solid electrolyte layer 28 to have higher oxygen partial pressure.Under this condition, when the gas atmosphere then provides an oxygen-lean gas having lower oxygen partial pressure causing the interface between the solid electrolyte layer 28 and the measurement electrode layer 30 to have lower oxygen partial pressure, there occurs migration of oxygen ions from one side of the electrolyte layer contacting the reference electrode layer 26 to the other side of the electrolyte layer contacting the measurement electrode layer 30, causing the generation of electromotive force (EMF) permitting occurrence of current flowing in a direction. Thereafter, the oxygen partial pressure at the interface between the reference electrode layer 26 and the solid electrolyte layer 28 is gradually lowered and finally no electromotive force is generated.Under this condition, when the gas atmosphere provides again the oxygen-rich gas, the oxygen partial pressure at the interface between the measurement electrode layer 30 and the solid electrolyte layer 28 becomes higher thereby causing migration of oxygen ions from the other side of the electrolyte layer 28 contacting the measurement electrode layer 30 to the one side of the same contacting the reference electrode layer 26, permitting generation of current flowing in the reverse direction. Accordingly, the pulsating electromotive force is generated in response to changes in oxygen concentration in the gas atmosphere. This principle will become more apparent as the description is made with reference to Fig. 9.

In embodiments of the oxygen sensor according to the present invention, it is suitable to use an electrically nonconductive inorganic material such as alumina, mullite, spinel, siiica or forsterite as the material for the partition layer 24 or 34. Further, it is also possible to use as the material of the partition layer a cermet. In using these materials, the partition layer 24 or 34 may be made to have either a dense structure impermeable to gases or a somewhat porous structure. In case of the partition layer 24 or 34 being a structurally basic member of the oxygen sensor, it is possible to use either a moulded compressed powder article of the abovementioned materials or a cut piece such as a piece which is provided by cutting a ceramic slip.In case of the solid electrolyte layer 28 or 36 being a structurally basic member of the oxygen sensor, the partition layer 24 or 34 can be provided by dipping method, printing method or plasma spraying method.

As the material of the oxygen ion conductive solid electrolyte layer 28 or 36, ZrO2 stabilized with CaO, Y2O3, SrO, MgO, ThO2, WO3 or Tea205; By203 stabilized with Nb205, SrO, WO3, Ta2O5 or Y2O3; and Y203 stabilized with ThO2 or CaO are usable. When the partition layer 24 or 34 is constructed to serve as the structurally basic member of the sensor, the solid electrolyte layer 28 or 36 may take the form of a thin film deposited on the partition layer 24 or 34 by sputtering, vacuum evaporation or an electrochemical process, or by firing or baking of a solid electrolyte paste applied onto the partition layer.When the solid electrolyte layer 28 or 36 is made to be a structurally basic member of the sensor, this layer 28 or 36 takes the form of a sufficiently thick plate obtained by machining a sintered body of a selected one of the abovementioned oxide systems.

The reference and measurement electrode layers 26 and 30 are each made of an electron conductive cermet. Such electron conductive cermet is preferably a cermet provided by sintering Au, Ag or Sic (which does not exhibit any catalytic action on oxidation reactions) with the ceramic material which forms each of the partition layer 24 or 34 and the solid electrolyte layer 28 or 36, or by sintering Ru, Pd, Rh, Os, Ir or Pt (which exhibits catalytic action on oxidation reactions) with the ceramic material of the partition layer 24 or 34 and the solid electrolyte layer 28 or 36, or by sintering an alloy of these platinum group metals with the ceramic material, or by sintering an alloy of one or more of these platinum group metals and one or more of nonmetalic elements with the ceramic material.

Experiment has revealed that the preferable mixing ratio of the ceramic and the metal for forming the cermet is represented by the following equation: volume of the ceramic used ~ x 100 < 30% volume of the cermet produced This equation is backed up by a phenomenon in which if the ceramic-cermet volume ratio is less than 3%, the beforementioned diffusion of atoms or ions at the respective interfaces defined by the layers 24 or 34, 26, 28 or 36 and 30 does not occur, while if the ceramic-cermet volume ratio is greater than 30%, the electron conductivity of the produced cermet is greatly decreased. The deposition of these cermet layers onto the corresponding layers is made by sputtering, vacuum evaporation, an electrochemical process or firing of a metal powder paint or paste.

In case of the oxygen sensor 1 OB of Fig. 3, a thin wire of Pt or W may be used as the heating member 32 embedded in the partition layer 24. Further, firing a metal powder paste may be used for formation of such heating member 32.

When a porous protective layer 38 for protecting the measurement electrode layer 30 is employed as in case of Fig. 1 OD, such layer 38 is made of calcium zirconate (CaO-ZrO2 system), alumina or spinel and can be formed by wetting the measurement electrode layer 30 with an aqueous dispersion of such an oxide material and baking the wetted article, or by plasma spraying of the oxide material.

The present invention will be further illustrated by the following examples.

Example 1 As an example of the oxygen sensor of the present invention, the sensor 1 OD shown in Fig. 5 was produced.

To produce such sensor 1 OD, a pair of platinum wires 22a and 22b (0.2 mm in diameter and 10 mm in length) were placed on an alumina green sheet 24a (9 mm in length, 5 mm in width, 0.7 mm in thickness) in a manner as is shown in Fig. 6a. As is shown by Fig. 6b, another alumina green sheet 24b (of the identical size of the sheet 24a) having a pair of through holes 22c and 22d (both 1 mm in diameter) formed therein was placed on the green sheet 24a to put therebetween the wires 22a and 22b and then these two green sheets 24a and 24b were pressed against each other.Then, a cermet paste composed of 70 wt% powdered cermet material (which contains 95 wt% Pt and 5 wt% Y203 ZrO2 (1 :19 mole ratio) or 11 volume% Pt and 89 volume% Y203-ZrO2 (1 :19 mole ratio)) and 30 wt% organic medium (such as lacquer thinner) was applied onto a major portion of the outer face of the green sheet 24b, that is the hatched area shown in Fig. 6c, by the employment of a printing technique and the cermet paste was air-dried to form a non-baked conditioned layer of the reference electrode layer 26.The thickness of the layer 26 was about 10 ,um. Then, an electrolyte paste 28 composed of 70 wt% powdered Y203-ZrO2 (1 9 mole ratio) and 30 wt% organic medium (such as lacquer thinner) was applied by a printing technique onto the prior formed layer 26 so that the peripheral portion of the electrolyte paste 28 extends to the outer face of the green sheet 24b as is shown in Fig. 6d, and the paste 28 was air-dried to form a non-baked conditioned layer of the solid electrolyte layer 28.The thickness of the layer 28 was about 20 Mm. Then the same cermet paste as that forming the layer 26 was applied by a printing technique onto the outer side of the layer 28, that is the hatched area shown in Fig. 6e, and as shown in Fig. 6f, a platinum paste composed of 70 wt% powdered platinum and 30 wt% organic medium (such as lacquer thinner) was applied to the through holes 22c and 22d of the alumina green sheet 24b, and then the cermet paste and the platinum paste were air-dried to form a nonbaked conditioned layer of the measurement electrode layer 30 and non-baked conditioned connecting portions 22e and 22f.The thickness of the layer 30 was about 10 ym. Then, the non-baked conditioned multi-layered article thus produced was baked in air for 2 hours at a temperature of about 1 4600C. After this process, powdered calcium zirconate (CaO-ZrO2 system) was deposited on the entire front face of the baked article by plasma spraying technique to form a porous protective layer 38, as is depicted by Fig. 6g. The thickness of the protective layer 38 was about 80 to 1 00 ym.

Example 2 T e oxygen sensor (which will be designated by 1 OE) of this example was produced by taking substantially the same production technique as in the case of Example 1 except for materials of the reference and measurement electrode layers 26 and 30. In Example 2, a cermet paste composed of 70 wt% powdered cermet material (which contains 95 wt% Pt and 5 wt% Awl203 or 13 volume% Pt and 87 volume% Awl203) and 30 wt% organic medium (such as lacquer thinner) was used for the materials of the reference and measurement electrode layers 26 and 30. The thickness of the non-baked conditioned layers for the layers 26 and 30 were each about 10 ,um.

Example 3 The oxygen sensor (which will be designated by 1 OF) of this example was a slight modification of the oxygen sensor 1 OB of Fig. 3. More specifically, the sensor of this example took such a construction as comprising the sensor 1 OB of Fig. 3 and the porous protective layer 38 of the sensor 1 OD of Fig. 5.

The production method of the sensor of this example will be described with reference to Figs. 8a to 8c.

To produce the sensor, a platinum paste composed of 70 wt% powdered platinum and 30 wt% organic medium (such as lacquer thinner) was applied, by a printing technique, onto an alumina green sheet (9 mm in length, 5 mm in width, 0.7 mm in thickness) in a manner to form a letter M as shown in Fig. 8a, and the M-shaped paste heap on the sheet 24c was air-dried to form a non-baked conditioned layer for the heating member 32 (see Fig. 3). Then, four platinum wires 22g, 22h, 22i and 22j (0.2 mm in diameter and 10 mm in length, each) were placed in parallel on the alumina green sheet 24c in a manner that the two outer wires 229 and 22j contact leg portions of the M-shaped paste heap 32, as shown in Fig. 8a. Then, as is seen from Fig. 8b, another alumina green sheet 24d having the same size as the sheet 24c and having four aligned through holes 22k, 221, 22m and 22n (0.6 mm in diameter, each) was placed on the sheet 24c with the wires 229 to 22j between sheets 24c and 24d and these two alumina green sheets 24c and 24d were pressed against each other.Then, the same cermet paste as that forming the layer 26 of the sensor 1 OD of Fig. 5, the same electrolyte paste as that forming the electrolyte layer 28 of the sensor 1 OD, and the same cermet paste as that forming the reference electrode layer were applied in order to the alumina green sheet 24d and then the non-baked conditioned multi-layered article thus produced was baked, and then powdered calcium zirconate (CaO-ZrO2 system) was deposited on the entire front face of the baked article by taking the same technique as in the case of Example 1. The resistance of the heating member 32 thus formed was about 1.6 Q at room temperature.

Conventional Oxygen Sensor In order to evaluate embodiments of the oxygen sensor according to the present invention, a conventionally used oxygen sensor (which will be designated by 1 0') having such a construction as comprising the sensor 10 of Fig. 1 and the porous protective layer 38 of Fig. 5 was prepared for the purpose of comparison therebetween panormance and durability. The production process of this conventional oxygen sensor was made by taking substantially the same technique as in the case of Example 1 except that in the conventional one, the material for forming the reference and measurement electrode layers 14 and 1 8 was a platinum paste composed of 70 wt% powdered platinum and 30 wt% organic medium (such as lacquer thinner).When air-dried, the non-baked conditioned layers for the electrode layers 14 and 1 8 showed their thicknesses of about 10 Mm. The application of the porous protective layer such as the layer 38 of Fig. 5 onto the entire front face of the baked multi-layered article having the construction of Fig. 1 was made by taking substantially the same technique as in the case of Example 1.

Oxygen Sensor Holder for Examination In order to accurately examine the performance and durability of the sensor of the invention and those of the conventional sensor 10' mentioned above, a holder 40 shown in Fig. 7 was prepared. The holder 40 comprises a stainless steel tube 42 having louver openings 42a through which a gas subjected to measurement passes. The oxygen sensor to be examined is set in the tube 42. The tube 42 is welded at its open end to an end of a stainless steel tube 44 in which an alumina cylindrical body 46 is coaxially disposed. The other end of the stainless steel tube 44 is welded to an end of another stainless steel tube 48 in a manner to be coaxial therewith. Within the tube 48 are disposed a rubber seal 50 and an alumina insulating body 52. Mounted about the tube 48 is a connecting nut 50 which is used for fixing the holder 40 to a suitable member.Suitable numbers of platinum wires 56 are passed through openings (no numerals) formed in the body 46, the seal 50 and the body 52 for electrically connecting the oxygen sensor in the tube 42 to an external measurement unit (not shown).

Experiment 1 In this experiment, the initial characteristics of the oxygen sensor 1 OD (Fig. 5) and the conventional oxygen sensor 10' thus produced were examined. The initial characteristics of these sensors were examined by the use of a first sample exhaust gas having air-fuel ratio of 15.5:1 (which is leaner than stoichiometric (14.7:1 in combustion of petrol)), and a second sample exhaust gas having air-fuel ratio of 14.0:1 (which is richer than stoichiometric).The holder 40 in which the sensor 1 OD or the conventional sensor 10' is set was disposed in a duct, and the two kinds of sample exhaust gases were passed through this duct alternately, maintaining the gas temperature at about 6000(=. With this arrangement, the electromotive forces generated by these sensors were measured.

Fig. 9 shows the result of this experiment. The solid line curve represents the result of the sensor 1 or, while the broken line curve represents the result s of the conventional sensor 10'. As will be observed from the graph of Fig. 9, the sensor 1 OD exhibited substantially the same performance as the conventional sensor 1 0' in connection with the electromotive force characteristic and the responsiveness. This means that usage of electron conductive cermet as the material of the reference and measurement electrode layers does not affect the electromotive force characteristic and the responsiveness of the oxygen sensor.

Experiment 2 In this experiment, the oxygen sensor 10D and the conventional oxygen sensor 1 0' were subjected to examination, also. A constant current DC power source was connected between the reference and measurement electrode layers 26 and 30 or 14 and 1 8 so as to enforcedly cause oxygen ions to flow in the solid electrolyte layer 28 or 1 6. The positive terminal of the DC power source was connected to the reference electrode layer 26 or 14 and the negative terminal of the source was connected to the measurement electrode layer 30 or 18, maintaining a constant current of 5 ,uA through the solid electrolyte layer 28 or 1 6 between the reference and measurement electrode layers 26 and 30 or 14 and 18.Similarly to the case of Experiment 1, these oxygen sensors 1 0D, 10' were disposed in the sample gas duct in which the beforementioned two kinds of sample exhaust gases were alternately passed, maintaining the gas temperature at about 6000C.

Fig. 10 shows the result of this experiment. The solid line curve represents the result of the sensor 1 OD, while the broken line curve represents the result of the conventional oxygen sensor 10'. As will be observed from the graph of Fig. 10, the sensor 1 OD exhibits substantially the same performance as the conventional sensor in connection with the electromotive force characteristic and the responsiveness.

This means that usage of the electron conductive cermet as the material of the reference and measurement electrode layers does not affect the electromotive force characteristic and the responsiveness of the oxygen sensor even though an external electrical power source is applied to the sensor.

Experiment 3 The oxygen sensor 1 OD and the conventional oxygen sensor 10' were subjected to an endurance test in an exhaust passage of an automotive internal combustion engine. The test was made on five sample sensors of each kind and these sample sensors were exposed to the exhaust gas emitted from an automotive internal combustion engine under full throttle. The exhaust gas thus emitted was richer than stoichiometric (:C06.5%) and the temperature of the gas was about 7400C.

Under this test, however, none of the five oxygen sensors 1 OD exhibited any appreciable change in bond between the cermet electrode layers 26 and 30 and the partition layer 24 and the solid electrolyte layer 28 even after about 1 50 hours had elapsed. While, it was found that in all of the conventional oxygen sensors 10', the reference and measurement electrode layers had come off or peeled off from the partition layer and the solid electrolyte layer within 50 hours from starting the test.

This fact means that the durability of the oxygen sensor 10D is greatly improved in comparison with the conventional sensor.

Experiment 4 In this experiment, the oxygen sensor 1 0E produced in Example 2 and the conventional oxygen sensor 10' were subjected to an endurance test and more particularly to a heat cyclic test. This test was made on five sample sensors of each kind and the sensors subject to examination were disposed in an exhaust tube of an automotive internal combustion engine which was operated taking alternately a first operation mode of 700 rpm (idling) for 4 minutes and a second operation mode of 4100 rpm for 6 minutes to produce low temperature exhaust gas and high temperature exhaust gas, alternately. The first operation mode of the engine caused the sensors to be heated at about 4000C while the second operation mode of the engine caused the sensors to be heated at about 8000 C.Under this test, however, none of the five oxygen sensors 1 OE exhibited any appreciable change in bond between the cermet electrode layers 26 and 30 and the partition layer 24 and the solid electrolyte layer 28 even after about 250 hours had elapsed. While, all of the conventional sensors 10' were damaged in that the reference and measurement electrode layers thereof had come off or peeled off from the partition layer and the solid electrolyte layer within 100 hours from the test starting. This fact means that the durability of the oxygen sensor 1 OE produced in Example 2 is greatly improved as compared with the conventional sensor 10'.

Experiment 5 In this experiment, the oxygen sensor 1 OF produced in Example 3 and a conventional sensor having a construction of the prior-mentioned conventional oxygen sensor 10' with the heating member 32 of FIg. 8a were subjected to measurement of electromotive force. The first and second sample exhaust gases which are mentioned in Experiments 1 and 2 were used. Similarly to the case of Example 2, a constant current DC power source was connected between the reference and measurement electrode layers of the oxygen sensor with the positive terminal connected to the reference electrode layer 26 or 14 and the negative terminal connected to the measurement electrode layer 30 or 18, maintaining a constant current of 5,uA through the solid electrolyte layer 28 or 1 6.A constant current of about 1 A was continuously supplied to the heating member 32 during the measurement. The measurement was made to the difference between the electromotive force generated when the sensor was exposed to the lean first sample exhaust gas and electromotive force generated when the sensor was exposed to the rich second sample exhaust gas, at temperature of these gases gradually increasing.

Fig. 11 shows the result of this experiment. The results of the oxygen sensor 1 OF and the conventional sensor are both represented by the solid line curve a. As will be observed from the curve a, both the sensor 1 OF and the conventional sensor exhibited excellent low temperature characteristics. For comparison, the same test was made on an oxygen sensor having the construction of the sensor 1 OF with omission of the heating member 32 and the conventional sensor with omission of the heating member 32. The result of this test is represented by the broken line curve b. As will be observed from the curve b, both of these sensors having no heating members exhibited substantially the same electromotive force characteristic in which the generation of the electromotive force begins when the gas temperature exceeds 4000C. These two line curves a and b thus mean that the use of the electron conductive cermet as the material of the reference and measurement electrode layers does not affect the performance of the oxygen sensor.

As will be appreciated from the foregoing description, the oxygen sensor having electron conductive cermet reference and measurement electrode layers exhibits great durability without sacrificing the performance of electromotive force.

Claims

1. An oxygen sensor comprising: a layer of an oxygen ion conductive solid electrolyte; a porous and electronically conductive reference electrode layer formed on and in intimate contact with one side of the electrolyte layer; a porous and electronically conductive measurement electrode layer formed on an in intimate contact with the other side of the electrolyte layer; and a partition layer of an electrochemically inactive material formed on and in intimate contact with the outer side of said reference electrode layer, at least one of said solid electrolyte layer and said partition layer having a porous structure permeable to fluids and being formed such that said reference electrode layer communicates with an exterior atmosphere only through said porous structure, one of said solid electrolyte layer and said partition layer being so constructed and formed to serve as a structurally basic member of said oxygen sensor, wherein cermet is used as the material of said reference and measurement electrode layers.

2. An oxygen sensor as claimed in Claim 1, in which said measurement electrode layer contains therein the same components as said solid electrolyte layer.

3. An oxygen sensor as claimed in Claim 1 or 2, further comprising a heating member embedded in said partition layer.

4. An oxygen sensor as claimed in any one of Claims 1-3, further comprising a DC power source electrically connected between said reference and measurement electrode layers so as to enforcedly cause oxygen ions to flow from one side of said electrolyte layer to the other side of the same.

5. An oxygen sensor as claimed in any one of Claims 1-4, further comprising a porous protective layer which covers intimately the exposed surfaces of said measurement electrode layer and said solid electrolyte layer.

6. An oxygen sensor substantially as hereinbefore described with reference to and as shown in Figs. 2 to 5, 6a to 6g and 8a to 8c.