GB2081908A - Method of producing solid electrolyte oxygen-sensing element of laminated structure with measuring electrode partially deposited from vapor phase - Google Patents

Abstract

A method of producing an oxygen sensing element of the concentration cell type in the form of a lamination of relative thin layers. The first step of the method is to prepare a lamination of a ceramic shield layer (12) which serves as a substrate, an inner or reference electrode layer (16), an oxygen ion conductive solid electrolyte layer (18) and an outer or measurement electrode layer (20) which is porously formed by sintering an electronically conducting powder, preferably a cermet powder, applied to the solid electrolyte layer surface in the form of paste. To improve the responsiveness of the oxygen sensing element, an additional measurement electrode layer (24), preferably not thicker than 0.5 microns, is formed on the sintered measurement electrode layer (20) by a physical vapor deposition technique such as sputtering, ion plating or vacuum evaporation by using a mask formed with an opening through which only the outer surface of the sintered measurement electrode layer is exposed. <IMAGE>

Description

SPECIFICATION Method of producing solid electrolyte oxygensensing element of laminated structure with outer electrode deposited from vapor phase This invention relates to a method of producing an oxygen sensing element of the concentration cell type, which takes the form of a lamination of relatively thin layers including an inner electrode layer, an oxygen ion conductive solid electrolyte layer and an outer electrode layer and is particularly suited to a device to detect the air/fuel ratio of a gas mixture supplied to a combustorsuch as an internal combustion engine based on the amount of oxygen contained in the exhaust gas.

The usefulness of oxgyen sensors of the concentration cell type that utilize an oxygen ion conductive solid electrolyte typified by zirconia containing a stabilizing oxide such as yttria or calcia has been well appreciated in various fields.

In the current automobile industry it has become popular to provide an oxygen sensor of this type to the engine exhaust system for the purpose of detecting fluctuations of 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 has a sintered solid electrolyte layer, a measurement electrode layer formed on one side of the solid electrolyte layer so as to be exposed to a gas subject to measurement and a reference electrode layer formed on the opposite side where a reference oxygen partial pressure is to be established.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 measurement electrode layer is exposed.

A recent trend is to construct this concentration cell in the form of a lamination of thin, film-like layers. For example, the solid electrolyte layer is made as thin as about 30 microns and each of the two electrode layers is made still thinner. The concentration cell of the laminated structure is mounted on a thin plate of a ceramic material, which plate is called a substrate or shield layer, such that the reference electrode layer is tightly sandwiched between the shield layer and the solid electrolyte layer. Usually the concentration cell part of an oxygen sensing element of this type, or the element as a whole, is coated with a porous protecting layer of a ceramic material.

The material of the reference electrode layer is a metal, usually platinum or its alloy, or an electronically conducting mixture of a certain metal and its oxide, such as a Ni-NiO mixture, which can serve also as the source of a reference oxygen partial pressure. A typical material of the measurement electrode layer is platinum, or its alloy, which acts as a catalyst. Also, it has been proposed to use an electrically conducting cermet.

Each of these two electrode layers is formed so as to have a microscopically porous structure usually through the steps of applying a paste containing a powdered electrode material onto the surface of the shield layer or the solid electrolyte layer by a screen-printing technique, drying the resultant paste layer and firing the unfinished laminate to achieve sintering of the electrode material particles contained in the paste layer. In most cases, the solid electrolyte too is formed by a similar process.

In our view, hitherto developed oxygen sensing elements of the above described laminated structure type are not yet fully satisfactory in their responsiveness, that is, the amount of time lag in responding to a change in the concentration of oxygen in the gas in which the element is disposed, particularly when used in automotive exhaust systems. We have recognized that the unsatisfactoriness in the quickness of response is largely attributed to the physical structure of the measurement electrode layer formed through the aforementioned firing process.

The firing must be performed at a considerably high temperature, for example at about 1500"C, to achieve sufficient sintering of the electrode material applied onto the solid electrolyte layer initially in the form of wet particles. Inevitably there occurs considerable growth of the crystalline particles of the electrode material subjected to sintering, so that the resultant measurement electrode layer is constituted of relatively coarse grains (in the microscopic sense) and therefore is not so large as expected in its effective surface area to make contact with a gas subject to measurement.Screen-printing is convenient to form a thin electrode layer of a desired pattern, but the employment of this technique results in that the measurement electrode layer after sintering has a relatively coarsely porous structure and accordingly is not so large in its effective surface area relative to its macroscopic surface area. For these reasons, the number and total area of the so-called triple-phase points, where the solid electrolyte, measurement electrode and the gas subject to measurement come into contact with each other, in the oxygen sensing element are not so large as expected relative to the macroscopic surface area of the measurement electrode layer. Therefore, the measurement electrode layer becomes unsatisfactory in its ability of promoting catalytic electrode reactions.For the oxygen sensing element having this measurement electrode layer, it takes a relatively large amount of time to establish an equilibrium oxygen partial pressure at the measurement electrode side of the concentration cell as the basis of generation of an electromotive force, so that the element fails to very quickly respond to a change in the oxygen concentration in, for example, an engine exhaust gas resulting from a change in the air/fuel ratio value of a gas mixture supplied to the engine.

It has been proposed to form a thin measurement electrode layer on the surface of a sintered solid electrolyte layer by using a physical vapor deposition technique, for example in U.S. Patent No.

3,978,006. An oxygen sensing element produced through this process is improved in its quickness of response. However, this oxygen sensing element is inferior in durability because the deposited measurement electrode layer is weak in the strength of adhesion to the sintered solid electrolyte layer and is liable to peel from the solid electrolyte layer during use of the oxygen sensing element under severe environmental conditions as in an automobile exhaust system.

It is an object of the present invention to provide an improved method of producing an oxygen sensing element of the above described laminated structure type having a sintered measurement electrode layer on the outer side of a solid electrolyte layer, which method is highly effective for improving the quickness of response of the oxygen sensing element to a change in the concentration of oxygen in a gas brought into contact with the measurement electrode and also for improving the durability of the oxygen sensing element in hot gas atmospheres.

In a method according to the invention for the production of an oxygen sensing element of the concentration cell type, the first step is to prepare a laminate which is constituted of a shield layer of a ceramic material, a reference electrode layer laid on a major surface of the shield layer, an oxygen ion conductive solid electrolyte layer formed on the reference electrode layer such that the reference electrode layer is tightly sandwiched between the shield layer and the solid electrolyte layer, and a measurement electrode layer which has a microscopically porous and gas-permeable structure and is formed on and in direct contact with the solid electrolyte layer.In the present invention this measurement electrode layer is referred to as first measurement electrode layer and is formed through the steps of applying a wet composition containing fine particles of an electrode material onto the outer surface of the solid electrolyte layer, drying the applied composition and firing the unfinished laminate to sinterthe electrode material particles contained in the applied composition. As the essential feature of the invention, the next step is to form a second measurement electrode layer which has a microscopically porous and gas-permeable structure on the first measurement electrode layer by physical vapor deposition of a metal by using a maskformed with an opening through which only the outer surface of the first electrode layer is exposed.

The physical vapor deposition of the second measurement electrode layer can be accomplished by sputtering, by ion plating or by vacuum evaporation. Preferably, the second measurement electrode layer is deposited to a thickness not greater than 0.5 microns.

For the second measurement electrode layer use is made of a metal that can catalyze oxidation reactions of carbon monoxide and hydrocarbons, as typified by platinum or its alloy with another metal of the platinum group.

The addition of the second measurement electrode layer formed by physical vapor deposition to the sintered first measurement electrode layer is effective for increasing the effective surface area of the catalytic measurement electrode and the aforementioned triple-phase points in the oxygen sensing element, and accordingly for promoting catalytic electrode reactions at the measurement electrode. Therefore, an oxygen sensing element produced by a method according to the invention becomes superior in the quickness of response to a change in the amount of oxygen contained in a gas atmosphere in which the element is disposed.

Accordingly, this oxygen sensing element is quite suitable for use as an exhaust gas sensor in a feedback type air/fuel ratio control system for an automotive internal combustion engine and, when put into such a use, can shorten the time lag in correcting a deviation of the air/fuel ratio from an intended value and, hence, can improve the accuracy of the control and facilitate purification of the exhaust gas. Besides, an improved responsiveness of this oxygen sensing element even at relatively low temperatures makes a contribution to an improvement in the controllability during a starting phase of operation of the engine under the control of the air/fuel ratio control system.

The second measurement electrode layer, which is formed solely on the sintered first measurement electrode layer and therefore makes contact with the solid electrolyte layer only through the pores in the first measurement electrode layer, adheres to the first measurement electrode layer with sufficiently high adhesion strength and exhibits good durability even when the oxygen sensing element is used under severe conditions as in an automobile exhaust system. Furthermore, the strength of adhesion between the first and second measurement electrode layers and the durability of the second measurement electrode layer, and accordingly the durability of the oxygen sensing element too, can greatly be improved by using an electronically conducting cermet as the material of the sintered first measurement electrode layer.

In the accompanying drawings: Figure 1 is a schematic and sectional view of an oxygen sensing element produced by a method according to the invention; Figure 2 shows the use of oxygen sensing element of Figure 1 in an air/fuel ratio detecting device; Figure 3 is a schematic and sectional view of an oxygen sensing element which resembles the element of Figure 1 but is the product of a method not in accordance with the invention; Figures 4(A) to 4(G) illustrate an exemplary process of producing the oxygen sensing element of Figure 1 by a method according to the invention; Figure 5(A) is a partial, sectional explanatorily enlarged view of an electrode layer formed by an intermediate step of a production method according to the invention; ; Figure 5(B) is a similar view of another electrode layer formed on the electrode layer of Figure 5(A) by the next step of the same production method; Figure 6 is an explanatory plan view of an electrode layer formed by an intermediate step of a production method which is generally in accordance with the invention but is slightly modified; Figure 7 shows a variation of the electrode layer of Figure 6 in a similar view; and Figure 8 is a chart showing variations in the performance of an automotive engine exhaust gas purifying system observed when several kinds of oxygen sensing elements produced by a method according to the invention and ones produced by different methods were alternately used in the purifying system.

Figure 1 shows a fundamental construction of an oxygen sensing element 10 produced by a method according to the invention. A structurally basic member of this element 10 is a base plate or substrate 12 which is made of an electrochemically inactive ceramic material. A reference electrode layer 16 is formed on a major surface of the substrate 12, and a layer 18 of an oxygen ion conductive solid electrolyte is formed on the same side of the substrate 12 so as to closely and substantially entirely cover the reference electrode layer 16. A first measurement electrode layer 20 is formed on the outer surface of the solid electrolyte layer 18, and a second measurement electrode layer 24 is formed on the outer surface of the first measurement electrode layer 20.In this example, the second measurement electrode layer 24 is somewhat smaller in surface area than the first measurement electrode layer 20 and patterned such that a marginal region 20a of the first measurement electrode layer 20 is left uncovered. Alternatively, the second measurement electrode layer 24 may be formed so as to cover substantially the entire area of the first measurement electrode layer 20 as illustrated in Figure 2, but it is a requisite that the second measurement electrode layer 24 never extends beyond the periphery of the first measurement electrode layer 20.

Each of the solid electrolyte layer 18 and the two electrode layers 16 and 20 is a thin, film-like layer (though regarded as a "thick film" in the field of current electronic technology), so that the total thickness of these three layers is, for example, only about 70 microns or even smaller. The second measurement electrode layer 24 is a thin film preferably not greater than 0.5 microns in its thickness. The substrate 12 may have a thickness of about 1 mm for instance. If desired, it is possible to make the 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 material.In view of such a possibility as well as a fact that macroscopically the reference electrode layer 16 is shielded from the environmental atmosphere by the substrate 12 and the solid electrolyte layer 18, in the present application the substrate 12 or a thin layer corresponding thereto is called a shield layer.

Preferably the outer surfaces of the multi-layered part of this element 10 are coated with a protecting layer 26 which is made of a ceramic material and has a porous structure to allow a gas subject to measurement to pass therethrough.

When it is intended to use the oxygen sensing element 10 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, a heater element 14 in the form of either a thin wire or a thin layer of an electrically resistive metal is embedded in the shield layer 12 because, as an inherent property of an oxygen ion conductive solid electrolyte, at relatively low temperatures the conductivity of oxygen ions in the solid electrolyte layer 18 becomes so low that the oxygen sensing element 10 cannot properly function. In that case, the shield layer 12 may be prepared by face-to-face bonding of two sheets 12a and 12b with the interposal of the heater element 14 therebetween.

Though not shown in Figure 1, electrical leads are connected to the reference electrode layer 16 and the first measurement electrode layer 20 to take out an electromotive force generated by an oxygen concentration cell constituted of the four layers 16, 18,20 and 24. The heater 14 is also provided with leads for the supply of a heating current.

The present invention does not place particular restrictions on the known materials and methods for the formation of the shield layer 12, reference electrode layer 16 and solid electrolyte layer 18.

Used for the shield layer 12 is a ceramic material such as alumina, mullite, spinel, forsterite or steatite.

The shield layer 12 to serve as the substrate of the element 10 is produced, for example, by sintering of a so-called green sheet prepared by moulding or extrusion of a wet composition containing a powder of 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.

Typical examples of electrically resistive metals for use as the heater 14 are platinum, tungsten and molybdenum. For example, the heater 14 can be embedded in the shield layer 12 as an assembly of the two sheets 12a and 12b by printing a paste containing platinum powder onto one of the two sheets 12a, 12b prior to bonding of these two plates and subsequently sintering the printed paste layer as well as the two plates 12a, 12b.

As to the material for the reference electrode layer 16, a choice is made between two categories of electrode materials depending on the method of establishing a reference oxygen partial pressure at the interface between this electrode layer 16 and the solid electrolyte layer 18. Where it is intended to establish the reference oxygen partial pressure without relying on any external measure, use is made 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 suitable amount of oxygen within the aforementioned concentration cell. Where it is intended to establish the reference oxygen partial pressure by supplying a DC current to the concentration cell in this element 10 such that a current of an adequate intensity flows through the solid electrolyte layer 18 to keep oxygen ions migrating through the solid electrolyte 18 between the reference electrode layer 16 and the measurement electrode layer 20 in a selected direction at an adequate rate, as proposed in U.S. Patent Nos. 4,207,159 and 4,224,113, the reference electrode layer 16 is formed of a metal, preferably selected from metals of the platinum group such as Pt, Ru, Pd, Rh, Os and Ir, alloys of these platinum group and alloys of a platinum group metal with a base metal. The reference electrode layer 16 is formed so as to have a microscopically porous structure permeable to gas molecules.

For example, the reference electrode 16 can be formed by applying a paste containing a powdered electrode material onto the outer surface of the shield layer 12 by a screen-printing method, drying the resultant paste layer and thereafter firing the dried layer.

The material for the solid electrolyte layer 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, WO3orTa205; Bi203stabilizedwith Nb205,SrO, WO3, Ta205 or Y203; and Y203 stabilized with ThO2 or CaO. In the case of the reference electrode layer 16 being formed of a metal-metal oxide mixture to serve as the source of an oxygen partial pressure, the solid electrolyte layer 18 is formed so as to have a tight structure practically impermeable to gases. In the case of establishing a reference oxygen partial pressure by the above described current-supplying method, the solid electrolyte layer 18 is formed so as to become microscopically porous and permeable to gas molecules.In the latter case, it is preferable to form the solid electrolyte layer 18 by applying a paste containing a powdered solid electrolyte mate rial onto the shield layer 12 which has been laid with the reference electrode layer 16, drying the resultant paste layer and firing the dried layer.

The first measurement electrode layer 20 is made to be microscopically porous and permeable to gas molecules. For this electrode layer 20, use is made of an electronically conducting material which is resistantto corrosion and can catalyze oxidation reactions of carbon monoxide, hydrocarbons, etc. To realize a porous structure, this electrode layer 20 is formed by applying a wet composition such as a paste containing a powdered electrode material onto the outer surface of the solid electrolyte layer 18, drying the resultant paste layer and thereafter firing the dried layer to achieve sintering of the electrode material particles. It is suitable to employ a screen printing technique for the application of the wet composition.

As is conventional, metals of the platinum groups and their alloys are suitable as the material of the first measurement electrode layer 20, and the use of platinum or its alloy with another metal of the platinum group as typified by Pt-Rh is preferable.

In the present invention, however, it is more preferable to use an electronically conducting cer met as the material of the first measurement electrode layer 20 in view of the adhesion strength and durability of the second measurement electrode layer 24 formed subsequently. As the metal compo nentofthe cermet for this purpose, it is most suitable to use platinum or its alloy with another metal of the platinum group. The ceramic compo nent of the cermet can be selected from various kinds of metal oxides that can make strong adhesion to the solid electrolyte layer 18 when sintered. Some examples of useful metal oxides are Fe2O3, NiO, Cr203, CuO, ZrO2, MgO, CaO, Y203, Al203 and TiO2, and it is possible to use a mixture of at least two kinds of metal oxides.Most preferably, the ceramic component of the cermet would be essentially similar to the material of the solid electrolyte layer 18, such as ZrO2-CaO, ZrO2-Y203, ZrO2-MgO or Y203-CaO. Considering that when the solid electrolyte layer and the measurement electrode layer 20 are sintered simultaneously there occurs mutual diffusion of the solid electrolyte and electrode materials across the interface between the two layers 18 and 20, it is understood that very strong adhesion can be established between the solid electrolyte layer 18 and the first measurement electrode layer 20 by using the solid electrolyte oxides as the ceramic component of the cermet electrode 20.

To form the first measurement electrode layer 20 of cermet by the aforementioned printing-firing process, use is made of a paste prepared by uniformly dispersing a powder mixture of the metal component and the ceramic component in an organic vehicle composed of an organic binder and a solvent. Preferably the proportion of the ceramic component to the metal component is controlled such that when sintered the ceramic component occupies 3 to 30% of the total volume of the cermet electrode layer 20.

Sintering of the shield layer 12, reference electrode layer 16, solid electrolyte layer 18 and first measurement electrode layer 20 should be effected before formation of the second measurement electrode layer 24. To meet this requirement, these four layers 12, 16, 18 and 20 may be fired individually, that is, each in a state still having an entirely exposed outer surface.Alternatively and rather preferably, these four layers 12, 16, 18, 20 (and if desired the heater element 14 too), or the upper three layers 16, 18,20, may be sintered simultaneously by first laminating these layers 12, 16, 18,20 (accurately, unfired layers respectively as intermediates of these four layers) on top of one another without firing any of them in the course of the laminating process and then subjecting the resultant laminate to a firing process which is carried out in the atmospheric air.

After formation of the sintered first measurement electrode layer 20, the uncompleted element is subjected to ultrasonic cleaning in an organic solvent as a preparatory step to the formation of the second measurement electrode layer 24.

An electronically conducting material forthe second measurement electrode layer 24 can be selected from the catalytic and noncorrosive metals and alloys as mentioned with respect to the first measurement electrode layer 20. It is preferable to use platinum or its alloy with another metal of the platinum group. This electrode layer 24 is made microscopically porous and permeable to gas molecules and, unlike the first measurement electrode layer 20, must be formed by a physical vapor deposition technique such as sputtering, ion plating or vacuum evaporation. The particulars of this electrode layer 24 will later be described more in detail.

The porous protecting layer 26 is formed of a ceramic material such as alumina, mullite, spinel or calcium zirconate by the employment of a plasma spraying method for instance.

A preferable range of the thickness of the sintered first measurement electrode layer 20 is from about 5 microns to about 20 microns, but the second measurement electrode layer 24 by vapor deposition is made far thinner and preferably not thicker than 0.5 microns. From a practical viewpoint, the minimum thickness of the second measurement electrode layer 24 is about 0.1 micron.

Figure 2 illustrates the application of an oxygen sensing element produced by a method according to the invention to a device for detecting the air/fuel ratio of an air-fuel mixture supplied to a combustor such as an automotive internal combustion engine by sensing the concentration of oxygen in the combustion gas or exhaust gas. The fundamentals of this air/fuel ratio detector are disclosed in U.S.

Patents Nos. 4,207,159 and 4,224,113, but the oxygen sensing elements in these U.S. Patents do not comprise an electrode layer corresponding to the second measurement electrode layer 24 according to the invention.

As a feature of this air/fuel ratio detecting device, a DC power source 28 is connected to the reference electrode 16 and the first measurement electrode 20 of the oxygen sensing element 10, in parallel with a voltage-measuring instrument 30 to measure an output voltage of the element 10, to force a DC current of an adequately controlled intensity (e.g.

about 10 microamperes) to flow through the solid electrolyte layer 18 between the two electrode layers 16 and 20 to thereby cause an adequate rate of migration of oxygen ions through the solid electrolyte layer 18 from selected one of the two electrode layers 16, 20 towards the other, while either conversion of oxygen molecules to oxygen ions or conversion of oxygen ions to oxygen molecules takes place at the catalytic measurement electrode layers 20,24 contacting the exhaust gas and a reverse change at the reference electrode layer 16 to which diffuses the exhaust gas through the micropores in the solid electrolyte layer 18.As a joint effect of the migration of oxygen ions and diffusion of oxygen molecules in the solid electrolyte layer 18, a reference oxygen partial pressure of a suitable magnitude can be established at the interface between the reference electrode layer 16 and the 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 18 from the measurement electrode layer 20 towards the reference electrode layer 16 to thereby establish and maintain a reference oxygen partial pressure of a relatively small magnitude at the aforementioned interface in the element 10.

When the oxygen sensing element 10 is designed so as to establish a reference oxygen partial pressure therein by utilizing a metal-metal oxide mixture as the material of the reference electrode layer 16, a device analogous in purpose to the device of Figure 2 is constructed without the provision of the DC power source 28. However, the device of Figure 2 is advantageous in being capable of exactly detecting numerical values of the air/fuel ratio of either a lean mixture or a fuel-rich mixture.

For comparison, Figure 3 shows an oxygen sensing element 40 which resembles the element 10 of Figure 1 and functions on the same principle but is produced by a hitherto employed method. On the outer side of the solid electrolyte layer 18 in this element 40, there is only one layer of porously sintered electrode 20 which corresponds to the first measurement electrode layer 20 in the element 10 of Figure 1. As described in the introductory part of the present specification, the oxygen sensing element 40 of Figure 3 is not fully satisfactory in its quickness of response to a change in the oxygen concentration in a gas that comes into contact with the sintered measurement electrode layer 20.

Example 1 Figures 4(A) to 4(G) illustrate a process employed in this example to produce the oxygen sensing element 10 of Figure 1 for use in an air/fuel ratio detecting device of the type as shown in Figure 2.

Referring to Figures 4(A) and 4(B), an alumina green sheet 12a (5 mm x 9 mm wide and 0.7 mm thick) and another alumina green sheet 12b, which was similar in material and dimensions to the former sheet 1 2a but was formed with three through-holes 31,33,35, (0.6 mm in diameter) were used to constitute an unfired shield layer 12'.As shown in Figure 4(A), three platinum wires 32,34, 36 (0.2 mm in diameter) were partly placed on the alumina green sheet 1 2a in an arrangement corresponding to the holes 31,33,35 of the other sheet 12b, and a paste prepared by dispersing 70 parts by weight of platinum powder in 30 parts by weight of a lacquer comprising a resin binder and an organic solvent was applied onto the surface of the same sheet 12a by a screen-printing technique to form a paste layer 14' which was elongate and meandering in plan view shape and terminated at the tip portions of the platinum wires 32 and 36.After drying of this paste layer 14', the bored sheet 12b was placed on the former sheet 12a such that the tip portions of the three wires 32, 34, 36 were located just beneath the three holes 31, 33, 35, respectively, as can be seen in Figure 4(B), and the two sheets 12a,12b in this state were stuck to each other by the application of a pressure of about 10 kg/cm2 to give the unfired shield layer 12', which had been provided with leads 32,34,36 and the platinum layer 14' to become the heater element 14 in Figure 1 through a subsequent firing process.

Then, the aforementioned platinum paste was applied onto an outer surface of the unfired shield layer 12' (the outer surface of the bored sheet 1 2b) by a screen-printing technique so as to form a paste layer 16' as shown in Figure 4(C). This paste layer 16' was made to locally extend to the hole 33 in the shield layer 12' to fill up this hole 33 with the platinum paste and dried before the next procedure.

Next, a solid electrolyte paste prepared by dispersing 70 parts by weight of powdered ZrO2-Y203 (95:5 mole ratio) in 30 parts by weight of a lacquer was applied onto the outer surface of the dried platinum layer 16' by screen-printing so as to form a paste layer 18' as shown in Figure 4(D) and dried. As the result, the platinum layer 16' was substantially entirely (except the elongate part extending to the hole 33: this part can be regarded as part of a lead) covered by the solid electrolyte layer 18', which had not yet been sintered.

A cermet paste was prepared by first mixing 95 parts by weight of platinum powder with 5 parts by weight of the aforementioned ZrO2-Y203 powder (by volume, the proportion of the platinum powder to the ZrO2-Y2O3 powder was 89:11) and then dispersing 70 parts by weight of the powder mixture in 30 parts by weight of a lacquer. This paste was applied onto the outer surface of the dried but unfired solid electrolyte layer 18' by screen-printing so as to form a cermet paste layer 20', as shown in Figure 4(E), which was made to locally extend to the hole 35 in the unfired shield layer 12' to fill up this hole 35 with the cermet paste.

After drying of the cermet paste layer 20', the laminate in the state of Figure 4(E) was subjected to a firing process which was carried out in the atmospheric air at a temperature of 1 5000C for a period of 2 hours to achieve simultaneous sintering of all the layers 12', 14', 16', 18' and 20'. As the result, the platinum layer 16', solid electrolyte layer 18' and cermet layer 20' turned respectively into the reference electrode layer 16, solid electrolyte layer 18 and first measurement electrode layer 20 in Figure 1. At the same time the unfired shield layer 12' turned into the rigid ceramic shield layer 12, and the platinum layer 14' in the shield layer turned into the heater 14.After the firing process, the reference electrode layer 16 had a thickness of 15-20 microns, and the solid electrolyte layer 18 was about 30 microns in thickness, and the first measurement electrode layer 20 was 10-15 microns.

Referring to Figure 4(F), the second measurement electrode layer 24 in Figure 1 was formed by depositing platinum on the surface of the sintered first measurement electrode layer 20 by sputtering, preceded by ultrasonic cleaning of the fired but uncompleted element in an organic solvent.

The already fired and cleaned element was placed in a conventional sputtering apparatus, using a mask formed with an opening so as to achieve the deposition of a platinum film in a pattern as shown by the hatched area in Figure 4(F). That is, a marginal area 20a of the first measurement electrode layer 20 was covered with the mask, and the opening area of the mask was smaller than the surface area of the first measurement electrode layer 20 by about 10%.

Accordingly, the deposited platinum came into contact with the solid electrolyte layer 18 only through the pores in the sintered measurement electrode layer 20.

Initially the vacuum chamber of the apparatus was pumped out to a pressure lower than 1 x 10-5Torr, and thereafter argon gas was introduced into the chamber until the pressure in the chamber became in the range from lx 10-3to5x 10-2 Torr. The purpose of creating the initial high vacuum was to minimize the influence of impurities and particularly of a residual gas on the quality of the deposited film.

The range ofvacuum, 1 x 10-3 to 5 x 10-2 Torr, after the introduction of Ar gas was determined because a greater magnitude of vacuum tends to offer difficulty to the occurrence of glow discharge and also because in a smaller magnitude of vacuum there arise various problems such as contamination of the deposited film with the residual gas and the introduced gas and lowering of the rate of deposition by reason of scattering of the sputtering atoms. The aforementioned range of vacuum is the mostfavor- able when the purity and adhesion strength of the deposited film and the productivity are collectively taken into consideration.

After the introduction of Ar gas in this manner, sputtering operation was carried out by the application of an electric power of 0.15-0.3 KW. In this example platinum was used as the target material in view of its good electronic conductivity and catalytic activity. A comparable material is a platinum alloy such as Pt-Rh. The sputtering operation was terminated when the thickness of the deposited film, i.e.

second measurement electrode layer 24 reached 0.4-0.5 microns.

Referring to Figure 4(G), the fabrication of the oxygen sensing element 10 was completed by plasma-spraying of a powder of spinel onto the front side outer surfaces of the element in the state of Figure 4(F) to form a gas-permeably porous protecting layer 26, which was 60-80 microns in thickness.

In this oxygen sensing element 10 the lead wires 34 and 36 were used to supply a DC current to the oxygen concentration cell formed in this element 10, and the leads 32 and 36 were used to supply a heating current to the heater 14. That is, the lead wire 36 was used as a ground lead common to the concentration cell and the heater 14.

Example 2 In accordance with Example 1 the steps illustrated in Figures 4(A) to 4(E) were performed, and the laminate in the state of Figure 4(E) was subjected to the firing process described in Example 1.

After ultrasonic cleaning in an organic solvent and drying, the fired laminate was placed in a conventional ion plating apparatus by using a mask formed with an opening similarly to the mask used in Example 1. Initially the vacuum chamber in the apparatus was pumped out to a pressure below 1 x 10-5 Torr to minimize a residual gas, and thereafter oxygen gas was introduced into the chamber until the pressure in the chamber became in the range from 1 x 10-3 to 5 x 10-2 Torr. It is possible to use an oxygen-argon mixed gas in place of pure oxygen gas. In either case the outer surface of the sintered first measurement electrode layer 20, including the surfaces in the micropores of this layer 20, and the solid electrolyte surface exposed in the micropores of the electrode layer 20 are cleaned and rendered activated by ion bombardment with oxygen ions originating in the introduced gas. Besides, the presence of oxygen in the chamber produces a faborable effect on the manner of growth of the deposited metal film 24.

It is preferable that the magnitude of vacuum after introduction of either 2 gas or O2-AR gas is within the range from 1 x 10-3 to 5 x 10-2 Torr because a higher vacuum tends to offer difficulty to the occurr ence of glow discharge whereas a smaller magni tudeofvacuum raises various problems such as oxidation contamination of the chamber by reason of an increased quantity of oxygen in the chamber, intrusion of the introduced gas into the deposited film and lowering of productivity by reason of scattering of the evaporated and ionized electrode material.

After the introduction of oxygen gas in this manner an electric field was produced in the chambey to cause glow discharge, and then the evapor atorfilament of platinum was energized to commence deposition of platinum film 24 on the sintered measurement electrode layer 20. The ion plating operation was terminated when the thickness of the deposited second measurement electrode layer 24 reached 0.4-0.5 microns.

The production of the oxygen sensing element 10 was completed by forming the porous protecting layer 26 by the method employed in Example 1.

Example 3 The steps illustrated by Figures 4(A) to 4(E) were performed in accordance with Example 1, and the laminate in the state of Figure 4(E) was subjected to the firing process described in Example 1.

After ultrasonic cleaning in an organic solvent, the fired but yet uncompleted element was placed in a conventional apparatus for vacuum evaporation operation, using a mask formed with an opening similarly to the mask used in Example 1. As a preparatory step the vacuum chamber of the apparatus was pumped out to a pressure below 1 x 10-5 Torr to minimize a residual gas. Then oxygen gas was introduced into the vacuum chamber until the pressure in the chamber became in the range from 1 x 10-3 to 5 x 10-2 Torr. This range of vacuum is preferable because a higher vacuum tends to offer difficulty to the occurrence ofglow discharge and also because a smaller magnitude of vacuum results in lowering of the efficiency of sputter-etching (described below) by reason of scattering of the molecules of the residual gas and the introduced gas.It is possible to use an oxygen-argon mixed gas in place of pure oxygen gas.

Prior to commencement of vacuum evaporation, an electric field was produced in the vacuum chamber containing oxygen gas to cause glow discharge to thereby accomplish sputter-etching of the outer surface of the sintered first measurement electrode layer 20 and the solid electrolyte surface exposed in the pores of the first measurement electrode layer 20. This sputter-etching treatment has the effect of enhancing the strength of adhesion of the second measurement electrode layer formed by the subsequent vacuum evaporation operation to the first measurement electrode layer 20.Because, certain impurities adhering to the first measurement electrode layer 20 and the solid electrolyte surface exposed in the pores of the electrode layer 20 even after the ultrasonic cleaning can be removed by renewal of these surfaces by the sputter-etching or bombardment with ions formed by ionization of the oxygen molecules in the gas introduced into the vacuum chamber, and at the same time these surfaces become more suitable in their surface roughness.

After the sputter-etching process the chamber was again pumped out to a pressure below 1 x 10-5 Torr in order to discharge the contaminating impurity matter struck out by the sputter-etching to thereby avoid intrusion of such impurity matter into the electrode layer to be formed by the subsequent vacuum evaporation operation. Thereafter the heaterforthe evaporator (platinum in this example) was energized to commence deposition of platinum on the etched surface of the first measurement electrode layer 20. The evaporation operation was terminated when the thickness of the deposited electrode layer 24 reached a thickness of 0.4-0.5 microns.

As the final step, the porous protecting layer 26 was formed in accordance with Example 1.

Example 4 This example was generally similar to Example 3.

As a sole difference, the first measurement electrode layer 20 was formed by using the platinum paste described in Example 1 in place of the cermet paste used in Examples 1 to 3.

The fundamental reasons for the provision of the second measurement electrode layer 24 according to the invention will be explained with reference to the explanatory illustrations in Figures 5(A) and 5(B).

The first measurement electrode layer 20 is formed through a firing process which is carried out at a temperature as high as about 1500 C to achieve complete sintering of the electrode material such as platinum or a cermet containing platinum applied onto the surface of the solid electrolyte layer 18 in the form of fine particles, and also complete sintering of the ceramic layers 12 and 18 when the laminate in the state of Figure 4(E) is prepared wholly in unfired state. This electrode layer 20 must be formed so as to have a gas-permeably porous structure, and the pores in this layer 20 are desired to be very small in their cross-sectional areas.During the high temperature sintering process, however, there occur coagulation of the electrode material particles and growth of crystalline grains under sintering to result in that the first measurement electrode layer 20 is constituted of considerably coarse grains with relatively wide gaps or pores 21 therebetween as illustrated in Figure 5(A). Therefore, a considerably large part of the surface area of the solid electrolyte layer 18 is left exposed in the pores 21 of this electrode layer 20. This means that the surface area of this electrode layer 20 effective for contacting with a gas subject to measurement is not so large as expected and that the number of the so-called triple-phase points, where the solid electrolyte 18, measurement electrode 20 and a gas subject to measurement come into contact with each other, becomes undesirably small relative to the macroscopical surface area of the electrode layer 20. For this reason, the measurement electrode layer 20 exhibits its catalytic ability only to a limited extent and, hence, fails to sufficiently promote electrode reactions that take place during use of the oxygen sensor element. As a consequence, the oxygen sensing element becomes unsatisfactory in the quickness of its response to a change in the composition of the external gas atmosphere.

Referring to Figure 5(B), the second measurement electrode layer 24 formed by a physical vapor deposition method is constituted of very small particles of the selected metal, which intrude into the pores 21 in the sintered measurement electrode layer 20 to cover a large part of the exposed areas of the solid electrolyte layer 18. In other words, only through the pores 21 in the sintered electrode layer 20 the solid electrolyte 18 becomes in direct contact with the second measurement electrode 24. Also the surface of each grain 20 of the sintered measurement electrode layer is interstitially covered with these particles 24 of the second measurement electrode layer.The addition of the second measurement electrode layer 24 having a structure as illustrated in Figure 5(B) to the first electrode layer 20 results in a great increase in the effective surface area of the electrode constituted of these two layers 20,24 and also a great increase in the number of the triple-phase points in the resultant oxygen sensing element. Therefore, the double-layered measurement electrode 20, 24 becomes highly effective for promoting the catalytic electrode reactions expected to take place during operation of the oxygen sensing element, and as a natural consequence this oxygen sensing element responds very quickly to changes in the composition of gases subject to measurement.

If attention is paid only to the quickness of response, it is conceivable to form a single measurement electrode layer by a vapor deposition technique directly on the surface of the solid electrolyte layer 18, omitting the sintered measurement electrode layer 20. For an oxygen sensor having a sintered tube of zirconia as the structurally basic member, such a method of forming a measurement electrode layer on the outer side has already been proposed. Actually, an oxygen sensing element produced in this way exhibits an improved responsiveness compared with a correspondingly designed element having a sintered measurement electrode layer, but there arises a serious problem that the durability of the electrode layerformed by vapor deposition in hot and temperature-varying gas atmospheres is so low that the oxygen sensing element becomes almost impracticable.That is, the strength of adhesion between the solid electrolyte layer and the measurement electrode layer deposited thereon is insufficient, so that the electrode layer tends to peel from the solid electrolyte layer during use of the element. Besides, in the case of providing the protecting layer 26 to the oxygen sensing element not having a sintered measurement electrode layer between the solid electrolyte layer and the measurement electrode layer formed by vapor deposition, it becomes difficult to attain sufficiently strong and durable adhesion of the protecting layer to the fundamental part of the oxygen sensing element.

Referring to Figures 6 and 7, also it is conceivable to form a sintered measurement electrode layer 20A or 20B having either a relatively large opening 1 9A our a plurality of relatively small openings 19B in its central region to leave the surface of the solid electrolyte layer 18 exposed in the opening 19A or openings 19B and then forming a second measurement electrode layer (not shown in Figures 6 and 7) by vapor deposition over the entire area of the sintered electrode layer 20A, 20B including the opening(s) 19A, 19B. However, this method is not recommendable either, because during use of the resultant oxygen sensing element the second measurement electrode layer is liable to peel from the solid electrolyte layer 18 in the opening(s) 19A, 19B and, as a consequence, tends to peel from the sintered electrode layer 20A, 20B too and/or causes peeling of an outer protecting layer.

Therefore, the present invention requires to first form the sintered measurement electrode layer 20 macroscopically having no openings and then form the second measurement electrode layer 24 using a vapor deposition technique only on the sintered electrode layer 20. That is, the second measurement electrode layer 24 is in contact with the solid electrolyte 18 only through microscopical pores 21 in the sintered measurement electrode layer 20, and macroscopically the surface area of the second measurement electrode layer 24 is smaller than, or equal to the surface area of the sintered electrode layer 20.In practice, a vapor deposition process to form the second measurement electrode layer 24 is performed by using a mask which is so apertured and positioned as to prevent the occurrence of deposition on the marginal area of the solid electrolyte layer 18 outside the periphery of the sintered measurement electrode layer 20.

In forming the second measurement electrode layer 24, there is a possibility of employing a sort of two-stage vacuum deposition method, wherein a first stage vacuum deposition operation is terminated before the deposited layer reaches an intended thickness and the second stage vacuum deposition operation is started after a sort of annealing of the initially deposited metal particles, usually a heat treatment at about 1000 C. The principal object of such a two-stage deposition process is to make the metal particles deposited by the first stage operation serve as a sort of nucleus at the second stage deposition operation to thereby obtain the second measurement electrode layer with a microscopically desirable structure and also with a very high strength of adhesion to the sintered measurement electrode layer. In the present invention, however, it - is not necessary to employ a two-stage deposition method to form the second measurement electrode layer 24. As explanatorily illustrated in Figures 5(A) and 5(B), the first measurement electrode layer 20 formed by the printing-firing process is a relatively coarsely porous layer and has a considerably rough surface in the microscopic sense.In a vapor deposition process to form the second measurement electrode layer 24, the microscopic roughness of the surface of the sintered electrode layer 20 serves as a sort of nucleus for the growth of the deposited film 24, and consequentially the deposited electrode layer 24 adheres sufficiently strongly to the sintered electrode layer 20 even when the deposition is completed by a single-stage deposition operation. As described hereinbefore, very strong adhesion between the sintered and deposited electrode layers 20 and 24 is further ensured by using a conducting cermet as the material of the sintered electrode layer 20.

In the present invention it is preferred that the thickness of the second measurement electrode layer 24 does not exceed 0.5 microns. When this electrode layer 24 is formed too thick, the rough outer surface of the sintered measurement electrode layer 20, including the surfaces in the pores 21, is almost completely covered with the tiny grains of the deposited electrode layer 24. In such a state, there occurs decrease in the effective surface area and the number of the triple-phase points contrary to the purpose of forming the second measurement electrode layer 24, with the result that the doublelayered measurement electrode 20,24 does not efficiently promote catalytic electrode reactions.

Therefore, it becomes difficult to really improve the responsiveness of the oxygen sensing element if the second measurement electrode layer 24 is formed to an excessively great thickness. So long as the object of sufficiently improving the responsiveness of the oxygen sensing element and affording good durabil ityto the same element can be accomplished, it is favorable to deposit the second measurement electrode layer 24 to a relatively small thickness for reducing the material cost and raising the productivity in the vapor deposition operation.

With a view to conducting comparative experiments to confirm the effects of the invention by using oxygen sensing elements produced in the foregoing examples, somewhat different oxygen sensing elements were produced by the following methods that were not in accordance with the invention.

Reference A The oxygen sensing element 40 of Figure 3 was produced by performing the steps illustrated in Figures 4(A) to 4(E) in accordance with Example 1, firing the laminate in the state of Figure 4(E) under the same firing condition as in Example 1 and then forming the porous protecting layer 26 by the method employed in Example 1 but without preceded by the deposition of the second measurement electrode layer 24. That is, the measurement electrode of this oxygen sensing element 40 was the sintered cermet electrode layer 20 alone.

Reference B The oxygen sensing element 40 of Figure 3 was produced generally similarly to Reference A, except that the sintered measurement electrode layer 20 was formed by using the platinum paste mentioned in Example 4.

Reference C This reference was generally identical with Example 3, but the first measurement electrode layer (20) of the cermet was formed in a pattern as shown in Figure 6, not in the pattern of Figure 4(E). The second measurement electrode layer 24 was formed in the same pattern as in Example 3, i.e. in the pattern of Figure 4(F). Accordingly, the second measurement electrode layer 24 formed in this reference was in direct contact with the outer surface of the solid electrolyte layer 18 in its rectangular central area.

Reference D This reference was similar to Reference C except that the first measurement electrode layer of the cermet was formed in a pattern as shown in Figure 7, so that the second measurement electrode layer 24 was in direct contact with the outer surface of the solid electrolyte layer 18 in the plurality of rectangu laropenings 19B of the sintered electrode layer 20B.

Reference E This reference was generally similar to Example 3, but the vacuum evaporation operation was performed by using a mask formed with such a large opening that the deposited second measurement electrode layer covered the entire surface area of the sintered measurement electrode layer 20 and extended into the marginal region, 18a in Figure 4(F), of the solid electrolyte layer 18.

Reference F The steps illustrated in Figures 4(A) to 4(D) were performed in accordance with Example 1, and the laminate in the state of Figure 4(D) was subjected to the firing process described in Example 1. That is, the solid electrolyte layer 18 was sintered with no electrode layer thereon. Then a platinum electrode layer corresponding to the second measurement electrode layer 24 in Figure 4(F) was formed directly on the outer surface of the sintered solid electrolyte layer 18 by the vacuum evaporation process described in Example 3, followed by the formation of the porous protecting layer 26 in accordance with Example 3.

Reference G Using a sintered solid electrolyte tube (ZrO2-Y20B) which was closed at one end and had a wall thickness of about 1 mm, an oxygen sensing element was produced by forming a reference electrode layer of platinum on the inner side of the tube by the paste-printing and subsequent firing process and depositing a measurement electrode layer of platinum on the outer surface of the tube by a vacuum evaporation method.

Several samples were produced for each of the oxygen sensing elements of the above described Examples and References, and they were subjected to the following experiments.

Experiment 1 The samples of the oxygen sensing elements were individually installed in an exhaust pipe of a 1.8-liter automotive gasoline engine as a component of an air/fuel ratio detecting device, and the fuel supply means for the engine was controlled by a feedbacktype control system which produced a fuel supply rate control signal based on the result of a comparison between the output voltage of the oxygen sensing element (indicative of actual air/fuel ratio in the engine) and a reference voltage corresponding to the stoichiometric air/fuel ratio taken as the aim of the control to correct deviations of the actual air/fuel ratio from the stoichiometric ratio. A catalytic converter containing a conventional three-way catalyst was attached to the exhaust pipe at a section downstream of the oxygen sensing element.The three-way catalyst had the ability of catalyzing both oxidation of CO and HC and reduction of NOx and worked most efficiency in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture.

The engine was operated according to an operating condition pattern specified to simulate running of the car in urban areas.

The chart of Figure 8 shows the CO, HC and NOx emission values measured in this experiment for the oxygen sensing elements of Examples and References. Each value shown in this chart is an average of actual data obtained for five samples of each kind of oxygen sensing element.

It is apparent that the oxygen sensing elements of Examples 2, 3 and 4 and References C, D, E and F gave the best results. A feature common to these oxygen sensing elements was the presence of a measurement electrode layer formed by a vapor deposition method. When the three-way catalyst exhibited such high conversion efficiencies, the control of air/fuel ratio must have been performed with high accuracy and, therefore, the oxygen sensing elements must have been excellent in responsiveness. Therefore, the result of this experi ment can be taken as an evidence of the suppositions that the effective surface area of the measure ment electrode as well as the number of the triple-phase points can greatly be increased by the employment of a vacuum deposition method and that the resultant measurement electrode is high in its ability of promoting catalytic electrode reactions.

Expreiment2 This experiment was an endurance test. The samples of the oxygen sensing elements were kept disposed in a stream of a simulated exhaust gas. The gas temperature was periodically varied within the range from 450"C to 8500C, and the concentration of CO in the exhaust was varied periodically within the range from 0.3% to 5.0%. The test was continued for 250 hours, and samples of the respective oxygen sensing elements were taken out of the gas stream at intervals of 50 hours and subjected to visual observation under magnifying glass.

After the lapse of initial 50 hours, the samples of Reference F (deposition of platinum electrode layer directly on the solid electrolyte layer, omitting the sintered measurement electrode layer) exhibited peeling of the porous protecting layer and the deposited measurement electrode layer in some areas. The samples of Examples and other References exhibited neither peeling of any layer nor any other defective changes. The results of the subsequent observation were as follows.

Samples of Example 2 Neither peeling of the protecting layer or the second measurement electrode nor any other defective change was observed even after the lapse of 250 hr from the start of the test.

Samples of Example 3 The same good results as the samples of Example 2.

Samples of Example 4 No change was observed after the lapse of 100 hr from the start of the test. After the lapse of another 50 hr, some cracks were found in the protecting layer, and the measurement electrode layer exhibited slight deterioration. After another 50 hr (200 hr from the start of the test), both the protecting layer and the second measurement electrode layer had peeled from the underlying layers.

Samples of ReferenceA ofReference A The same good results as the samples of Examples 2 and 3.

Samples of Reference B No change was observed after the lapse of 100 hr from the start of the test. After the lapse of another 50 hr and further 50 hr, these samples exhibited the same changes as the samples of Example 4.

Samples of References C, D andE After the lapse of 100 hr from the start of the test, peeling of the protecting layer was observed in areas where the deposited measurement electrode layer was in direct contact with the solid electrolyte surface. After the lapse of another 50 hr, the protecting layer had completely peeled, and the sintered measurement electrode layer exhibited some deterioration. After the lapse of 250 hrfrom the start of the test, the sintered measurement electrode layer had significantly deteriorated and partly peeled from the solid electrolyte.

Samples of Reference F After the lapse of 100 hr from the start of the test, both the protecting layer and the deposited measurement electrode layer had peeled from the underlying layers.

Samples of Reference G After the lapse of 100 hr from the start of the test, peeling of the protecting layer was observed in areas where the deposited measurement electrode layer was present directly on the solid electrolyte surface. - Afterthe lapse of another 50 hr, both the protecting layer and the measurement electrode layer had completely peeled from the underlying layers.

The results of this test demonstrate superiority in durability of the oxygen sensing element samples of Examples 2 and 3, i.e. ones having the deposited second measurement electrode layer only on the sintered measurement electrode layer of the cermet.

The samples of Reference A, which had the sintered measurement electrode layer of the cermet without deposition of the second measurement electrode layer thereon, was also superior in durability, but they were inferior in responsiveness as implied by the data in Figure 8. When platinum was used as the material of the sintered measurement electrode layer in place of the cermet, as represented by the samples of Example 4 and Reference B, the durability was tolerable but considerably lower than in the case of using the cermet. Also it is demonstrated that the deposition of a platinum measurement electrode layer directly on the surface of the sintered solid electrolyte layer (either a film or a tube) as represented by the samples of References C, D, E, F and G, results in unsatisfactorily low strength of adhesion between the deposited electrode layer and the solid electrolyte, and it was found that peeling of the deposited electrode layer is accompanied by peeling of the protecting layer.

When considered collectively, the results of Experiments 1 and 2 can be taken as a convincing evidence of superiority of oxygen sensing elements produced by a method according to the invention both in responsiveness and durability, especially when a conducting cermet is used as the material of the sintered first measurement electrode layer.

Claims (21)

1. A method of producing an oxgyen sensing element of the concentration cell type, the method comprising the step of: preparing a laminate constituted of a shield layer of a ceramic material, a reference electrode layer laid on a major surface of said shield layer, an oxygen ion conductive solid electrolyte layer laid on said reference electrode layer such that said reference electrode layer is tightly sandwiched between said shield layer and said solid electrolyte layer and a first measurement electrode layer which has a microscopically porous and gas-permeable structure and is formed on and in direct contact with said solid electrolyte layer through the steps of applying a wet composition containing fine particles of an electrode material onto the outer surface of said solid electrolyte layer, drying the applied composition and firing the unfinished laminate to sinner the electrode material particles contained in the applied composition; and forming a second measurement electrode layer which has a microscopically porous and gaspermeable structure on said first measurement electrode layer by physical vapor deposition of a metal by using a mask formed with an opening through which only the outer surface of said first measurement electrode layer is exposed.

2. A method according to Claim 1, wherein said second measurement electrode layer is formed so as to be not greater than 0.5 microns in thickness.

3. A method according to Claim 2, wherein the material of said second measurement electrode layer is a metal which is electronically conducting and can catalyze oxidation reactions of carbon monoxide and hydrocarbons.

4. A method according to Claim 3, wherein said metal is selected from the group consisting of metals of the platinum group and alloys of metals of the platinum group.

5. A method according to any one of Claims 1 to 3, wherein said electrode material is a cermet consisting of a metal component which comprises platinum and a ceramic component which comprises at least one metal oxide.

6. A method according to Claim 5, wherein said ceramic component of said cermet is essentially similar to the material of said solid electrolyte layer.

7. A method according to Claim 5, wherein the ceramic component of said cermet occupies 3 to 30% of the total volume of the fired first measurement electrode layer.

8. A method according to Claim 5, wherein the material of said second measurement electrode layer is essentially similar to said metal component of said cermet.

9. A method according to Claim 2, wherein said electrode material is a metal selected from the group consisting of metals of the platinum group and alloys containing at least one metal of the platinum group.

10. A method according to Claim 1, wherein said second measurement electrode layer is formed by a sputtering process.

11. A method according to Claim 10, wherein said sputtering process is performed in an inert gas atmosphere at a pressure in the range from 1 x 10-3 to 5 x 10-2 Torr.

12. A method according to Claim 1, wherein said second measurement electrode layer is formed by an ion plating process.

13. A method according to Claim 12, wherein said ion plating process is performed in an oxygencontaining gas atmosphere at a pressure in the range from 1 x 10-3to5 x 10-2 Torr.

14. A method according to Claim 1, wherein said second measurement electrode layer is formed by a vacuum evaporation process.

15. A method according to Claim 14, wherein said vacuum evaporation process includes a preparatory step of renewing the outer surface of said first measurement electrode layer by sputter-etching in an oxygen-containing gas atmosphere at a pressure in the range from 1 x 10-3 to 5 x 10-2 Torr.

16. A method according to Claim 1,further comprising the step of subjecting said laminate to ultrasonic cleaning in an organic solvent prior to the step of forming the second measurement electrode layer.

17. A method according to Claim 1,further comprising the step of forming a porous protecting layer at least on the outer surface of said second measurement electrode layer.

18. A method according to Claim 1, wherein the material of said reference electrode layer is a metal selected from the group consisting of metals of the platinum group and alloys containing at least one metal of the platinum group.

19. A method according to Claim 1, wherein the material of said reference electrode layer is an electronically conducting mixture of a metal and an oxide thereof.

20. A method according to Claim 1, substantially as herein described in any one of Examples 1 to 4.

21. An oxygen sensing element produced by a method according to any one of Claims 1 to 20.