GB2065889A

GB2065889A - Solid Electrolyte Oxygen- sensing Element of Laminated Structure

Info

Publication number: GB2065889A
Application number: GB8039284A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1979-12-18
Filing date: 1980-12-08
Publication date: 1981-07-01
Also published as: AU518795B2; DE3047248A1; FR2472187A1; IT8050377A0; SE8008921L; AU6520980A; CA1155179A; DE3047248C2; IT1146071B; JPS6034064B2; JPS5686348A; GB2065889B; FR2472187B1

Abstract

An oxygen sensing element (10) for use in a device to detect oxygen concentration in a gas atmosphere or to detect air/fuel ratio in an internal combustion engine based on the oxygen concentration in the exhaust gas. The element is a lamination of generally flat and relatively thin layers including a shield layer (12) of a ceramic material, first solid electrolyte layer (14), inner electrode layer (16), second solid electrolyte layer (18) and outer electrode layer (20) laid one upon another in the mentioned order. The second solid electrolyte layer (18) and the two electrode layers (16, 20) constitute a concentration cell. The first solid electrolyte layer (14) is formed of essentially the same material as the second solid electrolyte layer (18), so that there is no or little difference in thermal expansion coefficient between the two layers (14, 18) which tightly sandwich the inner electrode layer (16) therebetween. Hence, the inner electrode layer (16) does not tend to separate from either of the two solid electrolyte layers (14, 18) even when the element (10) is subjected to great and repeated changes in temperature.

Description

SPECIFICATION Solid Electrolyte Oxygen#ensing Element of Laminated Structure This invention relates to an oxygen sensing element for use in a device to detect the concentration of oxygen in a gas atmosphere or to detect the air/fuel ratio of a gas mixture supplied to, e.g., an internal combustion engine based on the amount of oxygen contained in the exhaust gas, which element takes the form of a lamination of generally flat and relatively thin layers including an oxygen ion conductive solid electrolyte layer with outer and inner electrode layers respectively laid on its two opposite surfaces to constitute an oxygen concentration cell and has a shield layer on the inner electrode side of the concentration cell.

The usefulness of oxygen sensors of the concentration cell type utilizing an oxygen ion conductive solid electrolyte, which is essentially an oxygen ion conductive metal oxide as typified by ZrO2 and is usually added with a small amount of at least one kind of stabilizing oxide such as CaO or Y203, has been well appreciated in various fields.

In the current automobile industries it has been popularized to provide an oxygen sensor of this type to the engine exhaust system to detect changes in the actual air/fuel ratio of an air-fuel mixture supplied to the engine based on the amount of oxygen contained in the exhaust gas.

The oxygen-sensitive part of the sensor comprises a layer of a sintered solid electrolyte, an outer electrode layer formed on one side of the solid electrolyte layer so as to be exposed to a gas subject to measurement and an inner electrode layer formed on the opposite side which is taken as a reference side. Essentially these three layers constitute an oxygen concentration cell which can generate an electromotive force between the two electrode layers depending on the magnitude of an oxygen partial pressure in the gas to which the outer electrode layer is exposed.

It has prevailed to form the solid electrolyte layer into the shape of a tube closed at its one end in order to expose only the outer side of the concentration cell to an exhaust gas flowing in an exhaust pipe. However, it has been recognized that this design is disadvantageous, for example, in that the fabrication of the solid electrolyte tube is troublesome and involves considerable cost, that the solid electrolyte tube is unsatisfactory in its physical strength when subjected to mechanical shocks and vibrations incidental to an automotive exhaust system in addition to thermal shocks attributed to variations of the exhaust gas temperature, and that it is very difficult to produce a desirably small-sized oxygen sensing element of this design.

With the view of eliminating such disadvantages of the tubular oxygen sensing element a recent trend is to construct a solid electrolyte oxygen concentration cell in the form of a lamination of generally flat and relatively thin layers. The solid electrolyte layer in an oxygen sensing element of this type is a film-like layer as thin as 10-20 microns by way of example, and each of the outer and inner electrode layers formed on the two opposite surfaces of the solid electrolyte layer is a still thinner film-like layer.

The concentration cell constituted of these three layers is mounted on a shield layer, which is made of a sintered ceramic material as typified by alumina and takes the form of a plate or sheet rigid and thick enough to serve as a structurally basic member or substrate of the oxygen sensing element, such that the inner electrode layer of the concentration cell is tightly sandwiched between the shield layer and the solid electrolyte layer of the cell. Electrical leads are connected to the inner and outer electrode layers, and usually the concentration cell part of the element, or the entire element, is coated with a porous protecting layer formed of a ceramic material.

The development of oxygen sensors or air/fuel ratio detectors utilizing an oxygen sensing element of such a lamination type has reached the stage of practical applications to automobiles.

In the course of extensive development work, we have recognized that the durability of an oxygen sensing element of this type is primarily governed by the strength of adhesion of the very thin electrode layers to the adjacent layer or layers and that, in the hitherto developed elements, the adhering strength between the inner electrode layer and the shield layer is not yet fully satisfactory from a practical viewpoint. The inner electrode layer is made of a metal such as platinum, whereas the shield layer is made of a ceramic such as alumina. Therefore, no chemical bonding is established at the interface between these two layers, but these two layers adhere to each other solely by physical adhesion even when these two layers are formed through a simultaneous sintering process.When used in an automobile exhaust system, the oxygen sensing element is kept disposed in the exhaust pipe and repeatedly exposed to an exhaust gas stream which has a considerably high temperature and a considerably high velocity in a general sense but undergoes great fluctuations in both the temperature and velocity. Moreover, the temperature of the element greatly lowers when the operation of the engine is stopped. As the oxygen sensing element is irregularly and repeatedly heated and cooled in the exhaust pipe, considerable strains are developed in the inner electrode layer because there is a great difference in thermal expansion coefficient between the metal employed as the electrode material and the ceramic material of the shield layer.As a consequence, there is the tendency of the inner electrode layer separating from the shield layer so that the oxygen sensing element fails to have a practically sufficient service life. Possibly, a great difference in thickness between the shield layer and the Inner electrode layer will be an additional factor of the development of strains leading to breakage of the adhesion between these two layers, and of course the adhesion breakage is accelerated by the mechanical shocks and vibrations produced by the operation of the engine and vehicle.

It is an object of the present invention to provide an oxygen sensing element of a laminated structure type, which element comprises a solid electrolyte oxygen concentration cell fundamentally similarly to the hitherto developed and above-described element and functions on the same principle but employs a novel technique to fixedly mount the cell on a shield layer to prevent the inner electrode layer of the cell from separating from either of the adjacent layers and therefore exhibits excellent durability even under severe conditions in an automobile exhaust system.

The present invention has achieved this object by interposing an additional solid electrolyte layer between the shield layer and the inner electrode layer of the aforementioned oxygen sensing element of the lamination type, using essentially the same material as the solid electrolyte layer present between the inner and outer electrode layers for this additional solid electrolyte layer, to allow the inner electrode layer to make a close contact with a solid electrolyte layer on either side thereof without making contact with the shield layer.

More specifically, an oxygen sensing element according to the invention comprises a shield layer made of a ceramic material, a first oxygen ion conductive solid electrolyte layer which is laid on and is in close contact with a major surface of the shield layer, an inner electrode layer which is laid on and is in close contact with the outer surface of the first solid electrolyte layer, a second oxygen conductive solid electrolyte layer which is laid on the aforementioned surface of the shield layer so as to closely cover the inner electrode layer substantially entirely and an outer electrode layer which is laid on and is in close contact with the outer surface of the second solid electrolyte layer.In this element, the second solid electrolyte layer and the inner and outer electrode layers constitute an oxygen concentration cell, and the material of the first solid electrolyte layer is at least essentially the same as the material of the second solid electrolyte layer.

Preferably, the inner electrode layer is formed such that a marginal region of the first solid layer is left uncovered along substantially the entire periphery of the inner electrode layer, and then the second solid electrolyte layer is formed such that a marginal region thereof makes a close contact with the marginal region of the first solid electrolyte layer, with the result that the inner electrode layer is substantially entirely enclosed by the two solid electrolyte layers.

In this oxygen sensing element, there is no or little difference in thermal expansion coefficient between the two solid electrolyte layers which tightly sandwich the inner electrode layer therebetween. Therefore, the inner electrode layer hardly tends to separate from either of these solid electrolyte layers even when the element is subjected to considerable and repeated changes in temperature. Since the first solid electrolyte layer formed directly on the shield layer is a ceramic layer, this solid electrolyte layer can be made to adhere to the ceramic shield layer with a far greater adhesion strength compared with the adhesion of a metal electrode layer with a ceramic shield layer. Accordingly this solid electrolyte layer does not tend to separate from the shield layer, either.Therefore, this oxygen sensing element exhibits excellent durability and has a satisfactorily long service life even when used in an automotive exhaust system.

In the accompanying drawings:~ Fig. 1 is a schematic and sectional view of an oxygen sensing element as an embodiment of the present invention; Fig. 2 shows a fundamental construction of an air/fuel ratio detector which utilizes an oxygen sensing element similar in fundamental construction to the element of Fig. 1; Figs. 3-5 show three kinds of modifications of the oxygen sensing element of Fig. 1, respectively, all as further embodiments of the present invention; Figs. 6(A) to 6(G) illustrate a process of producing an oxygen sensing element of the type as shown in Fig. 1 or 2; Fig. 7 is a longitudinal sectional view of a probe part of an air/fuel ratio detector which utilizes the element produced by the method of Figs. 6(A) to 6(G); Figs. 8(A) to 8(C) illustrate a process of producing an oxygen sensing element of the type shown in Fig. 3;; Figs. 9(A) to 9(D) illustrate a process of producing an oxygen sensing element which is similar in principle to the element of Fig. 1 or 2 but is not in accordance with the present invention; and Figs. 10(A) and 10(B) illustrate modifications of the process steps shown in Figs. 9(A) and 9(B) to produce an oxygen sensing element which is similar in principle to the element of Fig. 3 but is not in accordance with the present invention.

Fig. 1 shows the construction of an oxygen sensing element 10 as a first embodiment of the present invention. In this element 10 a structurally basic member is a base plate or substrate 12 which is made of an electrochemically inactive ceramic material. A thin, filmlike layer 14 of an oxygen ion conductive solid electrolyte is formed on a major surface of the substrate 12, and formed on the outer surface of this layer 14 is a first or inner electrode layer 16 which is also thin and film-like. This electrode layer 16 is designed so as not to cover a marginal region 1 4a of the solid electrolyte layer 14 substantially along the entire periphery of the electrode layer 16.There is another solid electrolyte layer 18 which is formed so as to cover the electrode layer 16 substantially entirely and, in its marginal region, to make a close contact with the initially uncovered marginal region 1 4a of the first solid electrolyte layer 14.

Accordingly the electrode layer 16 is substantially entirely enclosed by the two solid electrolyte layers 14 and 18. The two solid electrolyte layers 14 and 18 may be different in thickness (usually the second layer 18 is thicker than the first layer 14) but are made of the same material, at least essentially. A second or outer electrode layer 20 is formed on the outer surface of the solid electrolyte layer 18, and electrical leads 24 are connected to the inner and outer electrode layers 16 and 20 to measure an output voltage the element 10 generates when disposed in an oxygen-containing gas atmosphere by means of a suitable voitage-measuring device 26.

Each of the two electrode layers 16, 20 and two solid electrolyte layers 14, 18 is a thin, filmlike layer (though a "thick film" in the field of the current electronic technology), so that the total thickness of these four layers is, for example, about 50 microns or smaller. The substrate 12 may have a thickness of about 1 mm, for example.

If desired, it is possible to make the second solid electrolyte layer 18 thick and rigid enough to serve as a structurally basic member of the element. In that case, the "substrate" 12 can be replaced by a thin, film-like layer of a ceramic layer. In view of such a possibility, in the present application the substrate 12 or a thin layer corresponding thereto is called a shield layer.

Preferably, the outer surfaces of the multilayered part of this element 10 are coated with a protecting layer 22 which is made of a ceramic material and has a porous structure to allow a gas subject to measurement to pass therethrough and arrive at the outer electrode layer 20, which is also made porous.

The essential feature of the present invention is the existence of the first solid electrolyte layer 14 between the shield layer 12 and the inner electrode layer 16. Therefore, the inner surface of the inner electrode layer 16 is in close contact with the first solid electrolyte layer 14 which is essentially of the same material as the second solid electrolyte layer 18 lying on the outer surface of this electrode layer 16.

In conventional or hitherto proposed oxygen sensing elements of an analogous type there is no layer corresponding to the first solid electrolyte layer 14 in Fig. 1, so that the shield layer (12) is in direct contact with the inner surface of the inner electrode layer (1 6) while the solid electrolyte layer (18) covers the outer surface of the same electrode layer (16). The ceramic material of the shield layer (12) differs in thermal expansion coefficient from both the metallic material of the electrode layer (16) and the solid electrolyte (18) thereon, so that the electrode layer (16) tends to separate from the shield layer (12) during use of the oxygen sensing element in hot gas atmospheres, especially when the gas temperature fluctuates greatly.

In the element 10 according to the invention, there is practically no difference in thermal expansion coefficient between the two solid electrolyte layers 14 and 18 respectively lying on the inner and outer surfaces of the inner electrode layer 16, and, besides, these two solid electrolyte layers 14 and 18 are in close contact with each other in their marginal regions (1 4a). In other words, the inner electrode layer 16 is entirely embedded in a homogeneous solid electrolyte layer (14+18).Therefore, little strain in developed in the inner electrode layer 16 during use of the oxygen sensing element 10 in hot gas atmospheres, and, hence, the inner electrode layer 16 does not tend to separate from the surfaces of the solid electrolyte layers 14, 1 8 even when the element 10 is used in automotive engine exhaust gases which flow at considerably high velocities and exhibit great fluctuations in temperature.

The adhering strength between the shield layer 12 and the first solid electrolyte layer 14 becomes greater than the adhering strength between the same shield layer and a metallic electrode layer because the solid electrolyte 14 is also a ceramic material. The adhering strength between these two layers 12 and 14 can be maximized by forming these two layers 12 and 14 by first placing them one upon another each in an unfired state and then subjecting the two layers 12, 14 to a simultaneous sintering process, wherein there occurs diffusion of the ceramic material particles of each layer 12, 14 into the opposite layer 14, 12 with a resultant increase in the effective area of the contact face between the two sintered layers 12 and 14.

A still greater adhering strength is realized between the two solid electrolyte layers 14 and 1 8 which do not essentially differ from each other in chemical composition. Usually the same solid electrolyte material is used for these two layers 14 and 18. However, it is permissible that the solid electrolytes for the two layers 14, 1 8 comprise the same kind of oxygen ion conductive oxide (e.g. ZrO2) and the same stabilizing oxide (e.g. Y203) but are different in the amount of the stabilizing oxide (e.g. 3 mole % in one layer and 5 mole % in the other) or comprises the same oxygen ion conductive oxide but utilize different kinds of stabilizing oxides (e.g. Y203 for one layer and CaO for the other layer), insofar as the two solid electrolytes are practically similar in their thermal expansion coefficients. It is preferable to form these two solid electrolyte layers 14 and 18 through a simultaneous sintering process for the same reason as described above in regard of the shield layer 12 and the first solid electrolyte layer 14. Accordingly it is preferable to produce the multi-layered structure of the oxygen sensing element 10 by first placing the five layers 12, 14, 16, 18 and 20 (and also the protecting layer 22 if suitable) one upon another each in an unfired (not yet sintered) state and then sintering all the layers simultaneously by a firing process.

Fig. 2 illustrates the application of an oxygen sensing'element according to the invention to a device for detecting the air/fuel ratio of an air-fuel mixture supplied to an internal combustion engine by sensing the concentration of oxygen in the exhaust gas. The fundamentals of this air/fuel ratio detector are disclosed in U.S Patents Nos.

4,207,1 59 and 4,224,113, but the oxygen sensing elements in these U.S. Patents do not comprise a solid electrolyte layer corresponding to the first solid electrolyte layer 14 according to the present invention.

In the oxygen sensing element 10 in Fig. 2, each of the second solid electrolyte layer 1 8 and the two electrode layers 16 and 20 is made to have a microscopically porous structure permeable to gas molecules, whereas the shield layer 12 is made to have a tight structure impermeable to gases. The first solid electrolyte layer 14 may be either porous or nonporous.As a feature of this device, a DC power source 28 is connected to the inner and outer electrode layers 16 and 20 of the element 10 in parallel with the voltage measuring devide 26 to force a constant DC current of an adequately determined intensity (e.g. about 10 ,uA) to flow through the second solid electrolyte layer 18 between the two electrode layers 16 and 20 to thereby cause an adequate rate of migration of oxygen ions through this solid electrolyte layer 18 from selected one of the two electrode layers 1 6, 20 towards the other electrode layer 20, 1 6.As a joint effect of the migration of oxygen ions and diffusion of oxygen molecules through micropores in the solid electrolyte layer 18, a reference oxygen partial pressure of a suitable magnitude can be established at the interface between the inner electrode layer 16 and the second solid electrolyte layer 18. For example, where the engine is operated with a lean mixture having an air/fuel ratio higher than the stoichiometric ratio, the DC current is forced to flow through the solid electrolyte layer 18 from the outer electrode layer 20 towards the inner electrode layer 16 to thereby establish and maintain an oxygen partial pressure of a relatively small magnitude at the aforementioned interface in the element 10.

The existence of the second solid electrolyte layer 14 does not affect the function of the oxygen concentration cell constituted of the second solid electrolyte layer 18 and the two electrode layers 16 and 20. As described in U.S.

Patent No. 4,224,113, the device of Fig. 2 makes it possible to detect actual air/fuel ratio values of a mixture supplied to the engine without the need of using any extra source for a reference oxygen partial pressure.

Fig. 3 shows an oxygen sensing element 1 OA also as an embodiment of the present invention.

This element 1 OA is fundamentally similar to the element 10 of Fig. 1 and, as a sole modification in construction, employs a grille-like inner electrode layer 1 6A having a plurality of apertures 17 in place of the uniformly formed inner electrode layer 16 in the element 10 of Fig. 1. The grille-like electrode layer 1 6A is employed in order to allow the two solid electrolyte layers 14 and 18 to come into contact with each other not only in their marginal region (1 4a) but also in the apertures 17 of the electrode layer 1 6A by filling up the apertures 17 with the solid electrolyte during formation of the second solid electrolyte layer 18 (or the first solid electrolyte layer 14).As a natural consequence the inner electrode layer 1 6A is still more securely held in fixed position by the two solid electrolyte layers 14, 18, with further lowering of the possibility of its separating from either of the two solid electrolyte layers 1 4, 18. In the production of this element 1 OA, it is particularly desirable to form at least the two solid electrolyte layers 14, 18 and the inner electrode layer 1 6A by the employment of a simultaneous sintering process.

Of course, the oxygen sensing element 1 OA of Fig. 3 can be used in the air/fuel ratio detecting device as illustrated in Fig. 2.

Fig. 4 shows a further modification of the oxygen sensing element 1 OA of Fig. 3. That is, the oxygen sensing element 1 OB of Fig. 4 comprises a heater element 30 which is either a thin wire or a thin film of an electrically resistive metal embedded in the shield layer 12. Indicated at 32 are leads for supplying a heating current to the heater element 30. In other respects, this element 1 OB is identical with the element 1 OA of Fig. 3.

As illustrated, this oxygen sensing element 1 OB, too, is of use in and quite suitable to the air/fuel ratio detecting device described with reference to Fig. 2. As an inherent property of an oxygen ion conductive solid electrolyte, at relatively low temperatures the conductivity of oxygen ions in the second solid electrolyte layer 18 becomes so low that the oxygen sensing element cannot properly function. Therefore, there arises the need of providing an oxygen sensing element according to the invention with a heating means when it is intended to use the element even in relatively low temperature gas atmospheres as exemplified by the case of detecting the air/fuel ratios in an internal combustion engine even during a starting phase of the engine operation where the exhaust gas temperature is not sufficiently high.In such a case, the heater element 30 would be so designed as to maintain the second solid electrolyte layer 18 at a temperature of 500 7000 C. Of course the provision of the heater 30 as illustrated in Fig. 4 is not limited to the element 1 OA of Fig. 3 but may also be effected to the element 10 of Fig. 1.

Fig. 5 shows an oxygen sensing element 1 OC, also as an embodiment of the invention, which is fundamentally similar to the element 1 OA of Fig.

3 except that the uniformly formed outer electrode layer 20 in Fig. 3 is replaced by a grillelike outer electrode layer 20A having a plurality of apertures 21, which are filled up with the ceramic material of the protecting layer 22. By this modification, the outer electrode layer 20A is more securely held in fixed position and accordingly is less liable to separate from the second solid electrolyte layer 18. This element 1 OC, too, is applicable to the air/fuel ratio detector illustrated in Figs. 2 and 4. It will be understood that the grille-like outer electrode layer 20A of Fig. 5 may be employed also in the element 10 of Fig. 1 and/or may be employed jointly with the heater 30 shown in Fig. 4.

The shield layer 12 in an oxygen sensing element according to the invention is usually made of a ceramic material such as alumina, mullite, spinel, forsterite or steatite, and also it is possible to utilize a cermet, i.e. a bonded mixture of a ceramic material and a metal. The shield layer 12 to serve as the substrate of the oxygen sensing element is produced, for example, by sintering of a so-called green sheet prepared by moulding or extrusion of a wet composition comprising a powdered raw material for a selected ceramic material as the principal component, by sintering of a press-formed powder material or by machining of a sintered plate of a selected ceramic material.In the case of making the second solid electrolyte layer 18 serve as the substrate, the shield layer (12) may be formed as a relatively thin, film-like layer, for example, by a physical deposition technique such as sputtering, by plasma-spraying, or by the steps of printing a paste containing a powdered raw ceramic material onto the substrate and then sintering the printed paste layer.

The material for each solid electrolyte layer 14, 18 can be selected from oxygen ion conductive solid electrolyte materials used for conventional oxygen sensors of the concentration cell type.

Some examples are ZrO2 stabilized with CaO, Y203, SrO, MgO, ThO2, W03 or Ta203; BiO3 stabilized with Nb2O5, SrO, W03, Tea 205 or Y203; and Y203 stabilized with ThO2 or CaO. The meaning of the essential identicalness in material between the first and second solid electrolyte layers 14 and 18 in the present invention is as described hereinbefore.

Where the shield layer 12 is made to serve as the substrate, each solid electrolyte layer 14, 18 can be formed as a thin, film-like layer by a physical deposition technique such as sputtering or ion plating or by a process having the steps of printing a paste containing a powdered solid electrolyte material onto the substrate 12 or onto the inner electrode layer and then firing the pasteapplied substrate. In the case of making the second solid electrolyte layer 18 serve as the substrate, this layer 18 may be produced, for example, by sintering of a press-formed powder material or sintering of a green sheet comprising a powdered solid electrolyte material.

The inner and outer electrode layers 16 (1 6A), 20(20A) are each made of an electronically conducting material selected from electrode materials known as useful for conventional solid electrolyte oxygen sensors. Examples are metals of the platinum group, which catalyse oxidation reactions, of hydrocarbons, carbon monoxide, etc., such as Pt, Pd, Ir, Ru, Rh and Os, including alloys of these platinum group metals and alloys of a platinum group metal with a base metal; and some other metallic materials such as Au, Ag and SiC which do not catalyze the aforementioned oxidation reactions.Also it is possible to use a cermet comprising one of the above metals and a ceramic material, preferably an oxide serving as the principal component of the solid electrolyte layers 14, 18 in the same oxygen sensing element, particularly for the inner electrode 1 6 (1 6A) in view of a further enhanced adhering strength of such a cermet electrode layer to the solid electrolyte layers 14, 18. For an oxygen sensing element to be used in the manner as shown in Fig. 1, preferably the inner electrode layer 16 (1 6A) is formed of an electronically conducting mixture of a metal and its oxide, such as Ni-NiO, Co-CoO or Cr-Cr203, which serves as the source of a reference oxygen partial pressure at this electrode layer 16 ( 1 6A) in this oxygen sensing element 10.

Each electrode layer 16 (1 6A), 20 (20A) is formed on a solid electrolyte layer 14 or 18, for example, by a physical deposition technique such as sputtering or vacuum evaporation, by an electrochemical technique typified by plating or by printing of a paste containing a powdered electrode material followed by firing of the printed paste layer.

For the protecting layer 22, use may be made of a ceramic material such as alumina, spinel mullite or calcium-zirconate. The porous protecting layer 22 may be produced, for example, by plasma spraying, by sputtering, by printing of a paste containing a selected raw ceramic material (or application of the paste by a different technique) and subsequent firing of the printed paste layer, or by the steps of immersing the oxygen sensing element in a slurry of a selected powder material, drying the slurry adhered to the element and then firing the thus treated element.

Typical examples of electrically resistive metals for use as the heater element 30 are Pt, W and Mo. The embedding of the heater element 30 in the shield layer 12 can be achieved, for example, by fabricating the shield layer 12 by face-to-face bonding of two sheets with the interposal of a metal wire or preceded by formation of a thin metal layer on one of the two sheets through printing of a metal paste on that sheet.

Example 1 Figs. 6(A) to 6(G) illustrate a process employed in this example to produce an oxygen sensing element 40 which was fundamentally of the construction shown in Figs. 1 and 2 and designed so as to serve as an element of an air/fuel ratio detector of the type shown in Fig. 2.

Referring to Figs. 6(A) to 6(C), an alumina green sheet 41 (formed of a wet alumina-base composition, 5x9 mm wide and 0.7 mm thick) and another alumina green sheet 43 which was similar in material and dimensions to the former sheet 41 but was formed with two through-holes 43a and 43b (0.6 mm in diameter) were used to constitute a shield layer 42, which was to be fired at a later stage to become a structurally basic member of the oxygen sensing element 40. As shown in Fig. 6(A), two 0.2 mm platinum wires 45 were partly placed on the alumina sheet 41.

Then the bored sheet 43 was placed on the former sheet 41 such that the tip portions of the two wires 45 were located just beneath the two through-holes 43a and 43b respectively, as can be seen in Fig. 6(C), and the two sheets 41,43 in this state were adhered to each other by the application of a pressure of about 10 kg/cm2 to give a shield layer 42, which had already been provided with lead wires 45 but had not yet been fired.

A solid electrolyte paste was prepared by uniformly dispersing 70 parts by weight of powdered ZrO2-Y203 (95:5 mole ratio) in 30 parts by weight of a lacquer comprising a resin binder and an organic solvent. This paste was applied onto an outer surface (the outer surface of the bored sheet 43) of the unfired shield layer 42 by a screen-printing technique so as to form a paste layer as indicated in Fig. 6(C) by the hatched area. Drying of this paste layer gave an unfired solid electrolyte layer 44, which was 1012 microns in thickness.

Then, a platinum paste, which was a dispersion of 70 parts by weight of platinum powder in 30 parts by weight of a lacquer, was printed onto the outer surface of the unfired solid electrolyte layer 44 in a pattern as indicated in Fig. 6(D) by the hatched area. A marginal region 44a of the solid electrolyte layer 44 was left uncovered substantially along the entire periphery of a resultant platinum paste layer, though this paste layer was made to locally extend to one (43a) of the two holes 43a, 43b in the shield layer 42 to fill up this hole 43a with the platinum paste. This platinum paste layer was dried to obtain an inner electrode layer 46, which had not yet been fired and in this state was 6-7 microns in thickness.

Referring to Fig. 6(E), the aforementioned solid electrolyte paste was printed onto the outer surface of the unfired inner electrode layer 46 so as to cover the marginal region 44a of the first solid electrolyte layer 44, too, with this paste as indicated by the hatched area. Drying of the thus formed paste layer gave a solid electrolyte layer 48 which had not yet been fired and in this state was 20-22 microns in thickness. As the result, the unfired inner electrode layer 46 was substantially entirely (except the elongate part extending to the hole 43a: this part can be regarded as part of a lead) enclosed by the two solid electrolyte layers 44 and 48.

Next, the aforementioned platinum paste was printed onto the outer surface of the unfired solid electrolyte layer 48 in a pattern as indicated in Fig. 6(F) by the hatched area, followed by drying, to form an outer electrode layer 50 which had not yet been fired and in this state was 6-7 microns in thickness. In this step, the platinum paste layer (50) was locally extended to the hole 43b in the shield layer 42 to fill up this hole 43b with the platinum paste.

Then the five-layered article in the-state of Fig.

6(F) was entirely subjected to a firing process, which was carried out in the atmospheric air for a period of 2 hours at a temperature of 1 5000C to achieve simultaneous sintering of the five layers 42,44, 46, 48 and 50. By this firing process, the shield layer 42 became a rigid, ceramic (alumina) plate having a tight and gas-impermeable structure, whereas each of the two solid electrolyte layers 44, 48 and the two electrode layers 46, 50 became microscopically porous and permeable to gas molecules.

Referring to Fig. 6(G), the production of the oxygen sensing element 40 was completed by plasma-spraying of a calcium-zirconate (CaO~ ZrO2) powder onto the front-side outer surfaces of the fired element of Fig. 6(F) to form a gaspermeably porous protecting layer 52 which was 80-100 microns in thickness.

A practicable probe of an air/fuel ratio detector as shown in Fig. 7 was fabricated by using the oxygen sensing element 40 of Fig. 6(G). The element 40 was mounted on an alumina rod 56 by using a ceramic cement. The rod 56 was formed with two axial bores through which the lead wires 45 of the element 40 were extended outwards. The alumina rod 56 was tightly fitted into a tubular holder 58 of stainless steel, and a stainless steel hood 60 formed with apertures 61 was welded to the forward end of the holder 58 so as to enclose the oxygen sensing element 40 therein. A stainless steel tube 64 was welded to the rear end of the holder 58, and a nut 66 for attachment of the holder 58 to, for example, an exhaust pipe in an automobile was fitted around this tube 64 and welded to the rear end of the holder 58 and the outer surface of the tube 64.

The lead wires 45 were extended through this tube 64, and a front end portion of the interior of this tube 64 was filled with a synthetic rubber sealant 68 and the remaining portion with a ceramic insulator 69 which was essentially alumina powder. This air/fuel ratio detector was subjected to an evaluation test as will later be described.

Example 2 Referring to Figs. 8(A) to 8(C), produced in this example was an oxygen sensing element 40A which was fundamentally of the construction shown in Fig. 3 and designed so as to serve as an element of an air/fuel ratio detector of the type shown in Fig. 2.

The unfired alumina shield layer 42 shown in Fig. 8(A) was prepared by assembling together the two alumina green sheets 41, 43 and two platinum wires 45 described in Example 1 by the same procedure as in Example 1, and an unfired solid electrolyte layer 44 was formed on the unfired shield layer 42 by using the solid electrolyte paste described in Example 1 so as to have a thickness of 10-12 microns in the dried state.

Then, the platinum paste described in Example 1 was applied onto the outer surface of the unfired solid electrolyte layer 44 by a screenprinting technique so as to form a paste layer in a grille-like pattern as indicated in Fig. 8(A) by the hatched area. This paste layer was made to locally extend to the hole 43a in the shield layer 42 to fill up this hole 43a with the platinum paste. In this printing process, a marginal region 44a of the solid electrolyte layer 44 was left uncovered.

Drying of this platinum paste layer gave a grillelike inner electrode layer 46A having a multiplicity of apertures 47. In the dried and unfired state, this electrode layer 46A was 6-7 microns thick.

Referring to Fig. 8(B), another solid electrolyte layer 48 was formed on the unfired electrode layer 46A by printing of the aforementioned solid electrolyte paste so as to cover the outer surface of the electrode layer 46A and the marginal region 44a of the first solid electrolyte layer 44 and at the same time fill up the apertures 47 in the grille-like electrode layer 46A, followed by drying of the resultant paste layer. In the dried and unfired state, this solid electrolyte layer 48 was 20-22 microns in thickness. Then the aforementioned platinum paste was printed onto the outer surface of the unfired solid electrolyte layer 48, as indicated in Fig. 8(B) by the hatched area 50.In this printing process, the platinum paste layer 50 was made to locally extend to the hole 43b in the shield layer 42 to fill up this hole 43b with the platinum paste. After drying this paste layer, i.e. unfired outer electrode layer 50, was 6-7 microns in thickness.

Next, an alumina paste prepared by dispersing 70 parts by weight of alumina powder, which had been used for the preparation of the shield layer 42, in 30 parts by weight of lacquer was printed onto the front-side outer surfaces of the multilayered article of Fig. 8(C) in a pattern as indicated in Fig. 8(C) by the hatched area, followed by drying. to form a protecting layer 52A which had not yet been fired.

The oxygen sensing element 40A was finished by subjecting the six-layered article in the state of Fig. 8(C) to a firing process which was carried out in air for a period of 2 hours at a temperature of 1 5000C to achieve simultaneous sintering of the six layers 42, 44, 46A, 48, 50 and 52A. By this firing process, the shield layer 42 became a rigid ceramic (alumina) plate having a gasimpermeably tight structure, whereas each of the two solid electrolyte layers 44, 48, two electrode layers 46A, 50 and protecting layer 52A became microscopically porous and permeable to gases.

An air/fuel ratio detector of the construction shown in Fig. 7 was fabricated also by using the oxygen sensing element 40A of Fig. 8(C).

Reference 1 For the sake of comparison in the aforementioned evaluation test, an oxygen sensing element not in accordance with the present invention was produced by omitting the formation of the first solid electrolyte layer 44 from the process employed in Example 1.

Referring to Fig. 9(A), the platinum paste used in Examples 1 and 2 was printed onto the surface of the unfired shield layer 42, which had been prepared by the procedure described in Example 1, to form a paste layer in a pattern as indicated by the hatched area, with an extension to the hole 43a in the shield layer 42 to fill up this hole 43a with the paste. This paste layer was dried to give an inner electrode layer 76 which had not yet been fired and in this state was 6-7 microns in thickness.Then, a solid electrolyte layer 78 as shown in Fig. 9(B) was formed so as to cover the unfired electrode layer 76 substantially entirely, and also the outer surface of the shield layer 42 in an area surrounding the periphery of the electrode layer 76, by the employment of the solid electrolyte paste and the printing method used in Example 1 to thereby form the second solid electrolyte layer 78. In the dried but unfired state, this solid electrolyte layer 78 was 20-22 microns in thickness.

Next, an outer electrode layer 80 as shown in Fig. 9(C) was formed on the unfired solid electrolyte layer 78 by using the aforementioned platinum paste and the procedure described with respect to Fig. 6(F) in Example 1. In the dried but unfired state, this electrode layer 80 was 6-7 microns in thickness.

Thereafter the four-layered article in the state of Fig. 9(C) was subjected to the same firing process as performed in Example 1 (1500 C,2 hr) to achieve simultaneous sintering of the four layers 42, 76, 78 and 80. As the result, the shield layer 42 became a rigid ceramic plate having a gas-impermeable structure, whereas the solid electrolyte layer 78 and each of the electrode layers 76, 80 became microscopically porous and permeable to gases. The production of an exygen sensing element 70 shown in Fig. 9(D) was completed by forming a porous protecting layer 82, which was identical with the protecting layer 52 formed in Example 1 in the material (calciumzirconate), forming method (plasma-spraying) and thickness (80-100 microns).

Reference 2 Also for the comparison purpose, an oxygen sensing element not in accordance with the invention was produced by omitting the formation of the first solid electrolyte layer 44 from the process employed in Example 2.

Referring to Fig. 10(A), a grille-like inner electrode layer 76A was formed on the unfired shield layer 42 prepared in accordance with Example 1 by the electrode-forming procedure described with reference to Fig. 9(A) except for the change in the printing pattern. In the dried but unfired state, the electrode layer 76A was 6-7 microns in thickness. Next, a solid electrolyte layer 78 as shown in Fig. 10(B) was formed by the procedure described with reference to Fig. 9(B), with due care to fill up the apertures 77 of the unfired electrode layer 76A with the solid electrolyte paste. In the dried but unfired state, this solid electrolyte layer 78 was 20-22 microns in thickness.

Thereafter, the formation of an outer electrode layer corresponding to the electrode layer 80 in Fig. 9(C), the firing process to achieve simultaneous sintering of the shield layer, solid electrolyte layer and two electrode layers and the formation of a porous protecting layer were sequentially performed in exact accordance with the steps in Reference 1 described with reference to Figs. 9(C) and 9(D).

Evaluation Test The oxygen sensing elements of Examples 1 and 2 and References 1 and 2 were tested each in the form of the air/fuel ratio detector of Fig. 7. For each of these four kinds of oxygen sensing elements, five samples were subjected to the same test.

The test was an endurance test. An automotive gasoline engine was mounted on a bench test apparatus, and each sample of the air/fuel ratio detectors of Fig. 7 was attached to the exhaust pipe of the engine so as to allow the exhaust gas to flow into and pass through the interior of the apertured hood 60 of the detector. The engine was fed with an air-fuel mixture of which the air/fuel ratio was somewhat lower than the stoichiometric air/fuel ratio and operated under a full-throttle condition. Because of the employment of the fuel-rich mixture, the exhaust gas was very low in the concentration of oxygen and contained about 5% of carbon monoxide. The exhaust gas temperature at the location of the air/fuel ratio detector was constantly about 8300C.A constant DC voltage source was connected to the leads 45 of the air/fuel ratio detector to force a constant current of a predetermined intensity of flow through the solid electrolyte layer between the two electrode layers in the oxygen sensing element from the inner electrode layer toward the outer electrode layer to thereby establish a constant reference oxygen partial pressure of a relatively large magnitude at the interface between the inner electrode layer and the solid electrolyte layer, and a voltmeter was connected between the two leads 45 to measure an output voltage developed by the oxygen sensing element exposed to the exhaust gas.

During an initial stage of the endurance test, every sample of the air/fuel ratio detectors produced a stable output voltage accurately indicative of the air/fuel ratio of the mixture fed to the engine. However, first the output of the samples comprising the oxygen sensing element 70 produced as Reference 1 became unstable, and after continuation of the endurance test for 50 hours it was revealed that separation had occurred between the inner electrode layer 76 and the shield layer 42 in every one of the five samples of this type of oxygen sensing element 70. The samples comprising the oxygen sensing element of Reference 2 exhibited better endurance, but after continuation of the endurance test for 200 hours it was confirmed that separation had occurred between the inner electrode layer (grille-like) 76A and the shield layer 42 in every one of the five samples.

In contrast, all the samples comprising the oxygen sensing element 40 of Example 1 or the element 40A of Example 2 continued to exhibit stable function even after the lapse of 200 hours from the start of the endurance test. It was confirmed that the inner electrode layer 46 or 46A in each sample remained in close contact with the inner and outer solid electrolyte layers 44 and 48 with no appreciable separation therebetween. For the samples comprising the element 40A of Example 2, no separation had occurred between the grille-like inner electrode layer 46A and either of the two solid electrolyte layers 44 and 48 even when examined after continuation of the endurance test for 250 hours.

Claims

1. An oxygen sensing element of the oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte, the element comprising:- a shield layer made of a ceramic material; a first oxygen ion conductive solid electrolyte layer which is laid on and is in close contact with a major surface of said shield layer; an inner electrode layer which is laid on and is in close contact with the outer surface of said first solid electrolyte layer; a second oxygen ion conductive solid electrolyte layer which is laid on said surface of said first solid electrolyte layer so as to closely cover said inner electrode layer substantially entirely; and an outer electrode layer which is laid on and is in close contact with the outer surface of said second solid electrolyte layer; said second solid electrolyte layer and said inner and outer electrode layers constituting an oxygen concentration cell, the material of said first solid electrolyte layer being at least essentially the same as the material of said second solid electrolyte layer.

2. An oxygen sensing element according to Claim 1, wherein said inner electrode layer is formed such that a marginal region of said first solid electrolyte layer is left uncovered along substantially the entire periphery of said inner electrode layer, said second solid electrolyte layer being formed such that a marginal region thereof makes a close contact with said marginal region of said first solid electrolyte layer.

3. An oxygen sensing element according to Claim 2, wherein the material of said second solid electrolyte layer comprises a major amount of an oxygen ion conductive metal oxide as a principal component and a minor amount of at least one stabilizing oxide, the material of said first solid electrolyte layer comprising a major amount of said oxygen ion conductive metal oxide as a principal component and a minor amount of said at least one stabilizing oxide.

4. An oxygen sensing element according to Claim 3, wherein the material of said first solid electrolyte layer is the same as the material of said second solid electrolyte layer also in the proportion of said oxygen ion conductive metal oxide to said at least one stabilizing oxide.

5. An oxygen sensing element according to Claim 3, wherein the material of said first solid electrolyte layer is different from the material of said second solid electrolyte layer in the proportion of said oxygen ion conductive metal oxide to said at least one stabilizing oxide.

6. An oxygen sensing element according to Claim 1, wherein at least one of said inner electrode layer and said outer electrode layer is a grille-like layer having a plurality of apertures, said apertures being filled up with the material of a layer adjacent the grille-like layer.

7. An oxygen sensing element according to Claim 1, 2 or 3, wherein said inner electrode layer is a grille-like layer having a plurality of apertures, one of said first and second solid electrolyte layers being formed so as to intrude into and fill up said apertures of the grille-like inner electrode layer.

8. An oxygen sensing element according to Claim 3, wherein at least one of said inner electrode layer and said outer electrode layer is made of a cermet which consists essentially of a metal and said oxygen ion conductive metal oxide.

9. An oxygen sensing element according to Claim 1, wherein at least said shield layer, said first solid electrolyte layer, said inner electrode layer and said second solid electrolyte layer are formed through the steps of laying these four layers one upon another while each of these four layers is yet in an unfired state and firing these four layers in the multi-layered state to achieve simultaneous sintering of these four layers.

10. An oxygen sensing element according to Claims 1, or 2, wherein said shield layer is made thick enough to serve as a structurally basic member of the element, each of said first and second solid electrolyte layers and said inner and outer electrode layers being a thin, film-like layer.

11. An oxygen sensing element according to Claim 1 , further comprising an electric heater element embedded in said shield layer.

12. An oxygen sensing element according to Claims 1, or 11, further comprising a gaspermeably porous protecting layer which is formed of a ceramic material and covers at least outer surfaces of said outer electrode layer and said second solid electrolyte layer.

13. An oxygen sensing element according to Claims 1 or 2, wherein each of said second solid electrolyte layer and said inner and outer electrode layers has a microscopically porous and gas-permeable structure, said shield layer having a tight structure impermeable to gases.

14. An oxygen sensing element according to Claim 13, wherein each of said inner and outer electrode layers is made of a metal which can catalyze oxidation reactions of hydrocarbons and carbon monoxide.

15. An oxygen sensing element according to Claim 1, wherein said inner electrode layer is made of an electronically conducting mixture of a metal and an oxide of said metal, said mixture having the function of providing a reference oxygen partial within the oxygen sensing element.

16. A device to detect the air/fuel ratio of an air-fuel mixture supplied to a combustor, the device comprising: an oxygen sensing element to be disposed in a combustion gas discharged from the combustor, the element comprising a shield layer which is made of a ceramic material and has a tight structure impermeable to gases, a first oxygen ion conductive solid electrolyte layer which is laid on and in close contact with a major surface of said shield layer and has a microscopically porous and gas-permeable structure, an inner electrode layer which is laid on and in close contact with the outer surface of said first solid electrolyte layer and has a microscopically porous and gaspermeable structure, a second oxygen ion conductive solid electrolyte layer which is laid on said surface of said first solid electrolyte layer so as to closely cover said inner electrode layer substantially entirely and has a microscopically porous and gas-permeable structure, and an outer electrode layer which is laid on and in close contact with the outer surface of said second solid electrolyte layer and has a microscopically porous and gas-permeable structure, said second solid electrolyte layer and said inner and outer electrode layers constituting an oxygen concentration cell, the material of said first solid electrolyte layer being at least essentially the same as the material of said second solid electrolyte layer;; a DC power source connected to said inner and outer electrode layers to force a constant current of a predetermined intensity through said second solid electrolyte layer between said inner and outer electrode layers to cause migration of oxygen ions through said second solid electrolyte layer from a selected one of said inner and outer electrode layers towards the other electrode layer to thereby establish a reference oxygen partial pressure at the interface between said inner electrode layer and said second solid electrolyte layer; and voltage measuring means connected to said inner and outer electrode layers for measuring an output voltage produced by said oxygen sensing element.

17. An oxygen sensing element according to Claim 1, substantially as herein described with reference to any one of Figs.1,3,4 and 5 of the accompanying drawings.

18. An oxygen sensing element according to Claim 1, produced by a process substantially as herein described in Example 1 or 2 with reference to Figs. 6(A) to 6(G) or Figs. 6(A) to 6(C) and 8(A) to 8(C) of the accompanying drawings.