FR2472187A1 - ELEMENT DETECTING SOLID ELECTROLYTE OXYGEM IN LAMINATED STRUCTURE AND DEVICE USING THE SAME - Google Patents

Abstract

THE INVENTION RELATES TO AN OXYGEN DETECTOR ELEMENT OF THE OXYGEN CONCENTRATION CELL TYPE USING A SOLID ELECTROLYTE CONDUCTING THE OXYGEN ION. ACCORDING TO THE INVENTION, IT INCLUDES A CERAMIC SHIELDING LAYER 12, A FIRST LAYER 14 OF A SOLID ELECTROLYTE CONDUCTING OXYGEN ION, A LAYER FORMING INTERNAL ELECTRODE 16, A SECOND LAYER 18 OF A SOLID ELECTROLYTE CONDUCTING THE OXYGEN ION. 'ION OXYGEN, AN EXTERNAL ELECTRODE LAYER 20, LAYERS 18, 16 AND 20 CONSTITUTING AN OXYGEN CONCENTRATION CELL AND THE MATERIAL OF BOTH LAYERS 14 AND 18 BEING SUBSTANTIALLY THE SAME. THE INVENTION APPLIES IN PARTICULAR TO OXYGEN SENSORS FOR THE AUTOMOTIVE INDUSTRY.

Description

The present invention relates to an element detecting oxygen at

  use in a device for detecting oxygen concentration in the atmosphere of a gas or for detecting the air / fuel ratio of a gas mixture supplied, for example, to an internal combustion engine, based on the amount of oxygen contained in the exhaust gas, which element has the form

  a layer of generally flat and relatively

  thin layers comprising a layer of a solid electrolyte conducting the oxygen ion with outer and inner electrode layers respectively disposed on its two opposite surfaces to form an oxygen concentration cell, having a shielding layer

  the internal electrode side of the concentration cell.

  The utility of the concentration cell-type oxygen sensors using a solid electrolyte conducts the oxygen ion, which is essentially an oxide of a conductive metal of the oxygen ion represented by ZrO 2 and which is usually supplemented with a small amount of at least one kind of a stabilizing oxide such as CaO or

  Y20 3 has been well appreciated in various fields.

  In the automotive industry, it has become popular to provide an oxygen sensor of this type in 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 portion of the sensor comprises a layer of an agglomerated solid electrolyte, an outer electrode layer on one side of the solid electrolyte layer to be exposed to a measured gas and a layer forming an internal electrode on the other side, which is taken as the reference side. Essentially, these three layers constitute an oxygen concentration cell that can produce an electromotive force between the two electrodic layers depending on the magnitude of an oxygen partial pressure in the gas to which it is exposed.

  the outer electrode layer.

  The solid electrolyte layer was mainly formed in a tube closed at one end to expose only the outer side of the concentration cell to exhaust gas flowing in an exhaust pipe.

  However, it was recognized that this conception was

  for example, because the manufacture of the solid electrolyte tube is tedious and involves a considerable price, because the solid electrolyte tube is not satisfactory from the point of view of physical resistance when it is subjected to mechanical shocks and vibrations of an automobile exhaust system in addition to thermal shocks attributed to variations in exhaust gas temperature, and because it is very difficult to produce an oxygen-sensing element sufficiently

  small dimension, this design.

  In order to eliminate these disadvantages of the tubular oxygen-detecting element, a recent trend is to construct an oxygen concentration cell E in solid electrolyte in the form of a lamination of generally flat and relatively thin layers. The solid electrolyte in an oxygen-sensing element of this type is a film-like layer having, for example, only 10 to 20 F thick, and each of the outer and inner electrode layers formed on both opposite surfaces of the solid electrolyte layer is an even thinner film-like layer. The concentration cell consisting of these three layers is mounted on a shielding layer, made 9G0 into an agglomerated ceramic represented by alumina

  and which is in the form of a rigid plate or sheet sufficiently

  thickness of the oxygen-sensing element, so that the inner electrode layer of the concentration cell is sandwiched very closely between the shielding layer and the solid electrolyte layer of the cell. Electrical conductors are connected to the inner and outer electrode layers and usually the cell-concentrating portion of the element, or the entire element, is coated with a layer

  protective porous formed of a ceramic.

  The development of oxygen sensors or air / fuel ratio detectors using an oxygen detecting element of such a laminated type has reached the stage of practical applications for automobiles. In the course of intensive development work, it has been recognized that the durability of an oxygen-sensing element of this type is mainly governed by the strength of adhesion of the very thin electrode layers to the adjacent layer or adjacent layers and that, in the elements developed so far, the adhesion force between the inner electrode layer and the shielding layer was not entirely satisfactory from a practical point of view. The inner electrode layer is made of a metal such as platinum, while the shielding layer is made of a ceramic such as alumina. Therefore, no chemical bond is established at the interface between these

  two layers, but they adhere to each other only

  By physical adherence even when formed by a simultaneous agglomeration process. When used in an automobile exhaust system, the oxygen sensing element is held in the exhaust pipe and exposed. repeatedly to an exhaust gas stream at a considerably elevated temperature and a considerably fast speed in a general sense, but undergoing severe fluctuations both in temperature and speed. Moreover, the temperature of the element drops

  strongly when the operation of the motor stops.

  Since the oxygen sensing element is repeatedly and unevenly heated and cooled in the exhaust pipe, considerable stresses develop in the inner electrode layer because there is a large difference in coefficient of thermal expansion between the metal used as the material of the electrode and

  the ceramic material of the shielding layer. As a result

  Accordingly, the inner electrode layer tends to separate from the shielding layer so that the oxygen sensing element can not have a sufficient useful life in practice. Optionally, a large difference in thickness between the layer of. shielding and the

  inner electrode layer will be an additional factor

  the development of the stresses leading to a rupture of the adhesion between these two layers, and of course, the rupture of the adhesion is accelerated by the mechanical shocks and the vibrations due to the operation.

engine and vehicle.

  The subject of the present invention is a detection element

  - the oxygen type laminated structure, which

  includes an oxygen concentration cell in electro-

  solid lyte, fundamentally similar to the element developed so far and described above, and which works on the same principle, but employing a new technique to securely mount the cell

  on a shielding layer to prevent the layer from forming

  The inner electrode of the cell separates from adjacent layers and therefore has excellent durability even under severe conditions.

  in an automobile exhaust system.

  In the present invention, this goal is achieved

  by interposing an additional solid electrolyte layer

  between the shielding layer and the layer forming

  internal electrode of the oxygen-sensing element

  mentioned top of the laminated type, using essential-

  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 be in close contact with an electrolyte layer. solid on each of its sides, without having

  contact with the shielding layer.

  More particularly, an oxygen sensing element according to the present invention comprises a shielding layer made of a ceramic, a first solid conducting electrolyte layer of the oxygen ion disposed on and in close contact with a major surface of the shielding, an inner electrode layer disposed on and in close contact with the outer surface of the first

  solid electrolyte layer, a second layer of electro-

  an oxygen ion conductive solid lyte disposed on the above-mentioned surface of the shielding layer to very precisely and substantially completely cover the inner electrode layer and an outer electrode layer which is disposed on and in close contact with the surface outer layer 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 substantially the same as that of the

  second layer of solid electrolyte.

  Preferably, the inner electrode layer is formed so that a marginal region of the first solid electrolyte layer is left uncovered substantially all around the inner electrode layer, and then the second solid electrolyte layer is formed. so that its marginal region is in close contact with the marginal region of the first solid electrolyte layer, with the result that the inner electrode layer is substantially completely enclosed between the two solid electrolyte layers. In this oxygen sensing element, there is no or little difference in coefficient of thermal expansion between the two solid electrolyte layers that take up

  in a very tight sandwich the inner electrode layer.

  As a result, the inner electrode layer hardly tends to separate from the solid electrolyte layers

  even if the element is subject to considerable changes

  and repeated temperature. Since the first solid electrolyte layer formed directly on the shielding layer is a ceramic layer, this solid electrolyte layer may be forced to adhere to the ceramic shield layer with much better adhesion strength than that obtained between a layer forming

  metal electrode and a ceramic shield layer.

  As a result, this solid electrolyte layer does not have

  nor does it tend to separate from the armor layer.

  Therefore, this oxygen sensing element has excellent durability and has a sufficiently long life even when used in a system.

automobile exhaust.

  The invention will be better understood and other purposes,

  characteristics, details and advantages of it

  more clearly in the explanatory description

  which will follow made with reference to the accompanying schematic drawings given solely by way of example illustrating several embodiments of the invention and in which: - Figure 1 is a schematic and sectional view

  of an element detecting oxygen according to a mode of realization

  of the present invention; FIG. 2 shows a fundamental construction of an air / fuel ratio detector using an oxygen sensing element similar in construction to that of FIG. 1;

  - Figures 3 to 5 show three kinds of changes

  tiorsde the oxygen detecting element of Figure 1, respectively, according to other embodiments of the present invention; FIGS. 6A to 6G illustrate a method of producing an oxygen sensing element of the type illustrated in FIG. 1 or FIG. FIG. 7 is a longitudinal sectional view of a probe portion of an air / fuel ratio detector which uses the element produced by the process of FIGS.

6A to 6G;

  FIGS. 8A to 8C illustrate a method of producing an oxygen detecting element of the type shown in FIG. 3;

  FIGS. 9A to 9D illustrate a production process

  an oxygen-sensing element which is similar in principle to the element of FIG. 1 or FIG. 2, but which is not according to the invention; and FIGS. 10A and 10B illustrate modifications of the steps of the method shown in FIGS. 9A and 9B to produce an oxygen sensing element similar in principle to the element of FIG.

is not according to the invention.

  Figure 1 shows the construction of an oxygen sensing element 10 according to a first embodiment of the invention. In this element 10, a base plate or substrate 12 of an electrochemically inactive ceramic material forms a basic structure member. A thin film-like layer 14 of a solid electrolyte conducting the oxygen ion is formed on a major surface of the substrate 12, and on the outer surface of this layer 14 is formed a first inner electrode layer 16qii is also thin and film-like. This electrode layer 16 is designed so as not to cover a marginal region 14a of the solid electrolyte layer 14, substantially all around the electrode layer 16. Another solid electrolyte layer 18 is formed so as to covering the electrode layer 16 substantially completely and so as to be, by its marginal region, in close contact with the initially uncovered marginal region 14a of the first solid electrolyte layer 14. Accordingly, the electrode layer 16 is substantially completely enclosed by the two solid electrolyte layers 14 and 18. The two layers 14 021t287

  and 18 may have different thicknesses (usually

  the second layer 18 is thicker than the first layer 14) but they are made of the same material, at least essentially. A second outer electrode layer 20 is formed on the outer surface of the solid electrolyte layer 18, and electrical conductors 24 are connected to the inner and outer electrode layers 16 and 20 to measure the output voltage that

  element 10 produced when it is disposed in an atmosphere

  of a gas containing oxygen, by means of a

  suitable device 26 for measuring voltage.

  Each of the two electrode layers 16, 20 and the two solid electrolyte layers 14, 18 is a thin film-like layer (although a "thick film" in the field of current electrolyte technology), and total thickness of these

  four layers is, for example, of the order of 50 p or less.

  The substrate 12 may have a thickness of the order of 1 mm, for example. If desired, it is possible to form the second solid electrolyte layer 18 sufficiently rigid and thick to serve as a basic structural member of the element. In this case, the "substrate" 12 can be replaced by a thin film-like layer of a ceramic. Given these possibilities,

  in the present description, the substrate 12 or a

  thin layer corresponding to it is called shielding layer. Preferably, the outer surfaces of the multilayered portion of this member 10 are coated with a protective layer 22 made of a ceramic and which has a porous structure to allow a measured gas to pass therethrough and arrive on the layer

  external electrode 20, which is also porous.

  The essential feature of the present invention is the existence of the first solid electrolyte layer 14 between the shielding layer 12 and the inner electrode layer 16.

2472187.

  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 on the outer surface of this electrode layer 16. In the traditional oxygen-sensing elements proposed to date, of a similar type, there is

  no layer corresponding to the first layer of electro-

  solid layer 14 of Figure 1, and the shielding layer 12 is in direct contact with the inner surface of the inner electrode layer 16 while the solid electrolyte layer 18 covers the outer surface of the same layer 16. The ceramic of the shielding layer 12 differs, by its coefficient of thermal expansion> of the metallic material of the electrode layer 16 and the solid electrolyte 18 from above, so the electrode layer 16 tends to separate from the shielding layer 12 during use of the oxygen sensing element in hot gas atmospheres, in particular

  if the temperature of the gases fluctuates strongly.

  In the element 10 according to the invention, there is practically no

  there is no difference in coefficient of thermal expansion between the two solid electrolyte layers 14 and 18 respectively on the inner and outer surfaces of the inner electrode layer 16 and, in addition, these two solid electrolyte layers 14 and 18 are in close contact with each other in the marginal region (14a). In other words, the inner electrode layer 16 is completely embedded in a

  homogeneous solid electrolyte layer '4 + 18'.

  As a result, little stress develops in the inner electrode layer 16 during use of the oxygen sensing element 10 in hot gas atmospheres, and therefore the inner electrode layer 11 does not tend to separate. surfaces of the solid electrolyte layers 14 and 18 even if the element is used in the exhaust gases of an engine

2472187.

  cars that are flowing at considerable speeds

  are high and have high temperature fluctuations. The adhesion strength between the shielding layer 12 and the first solid electrolyte layer 14 is greater than the adhesive force between the same shielding layer and a metal electrode layer,

  because the solid electrolyte 14 is also ceramic.

  The adhesion strength between these two layers 12 and 14 can be maximized by forming these two layers 12 and 14 by first placing them on top of one another in an uncured state and then subjecting the two layers 12 and 14 to a simultaneous agglomeration process, during which there occurs a diffusion of the ceramic particles of each layer 12, 14 in the opposite layer 14, 12 with resulting increase in the effective area of the face

  contact between the two agglomerated layers 12 and 14.

  An even greater adhesive force can be achieved between two solid electrolyte layers 14 2 () and 18 which do not substantially differ from each other in their chemical composition. Usually, the same solid electrolyte material is used for these two layers 14 and 18. However, it is possible that the solid electrolytes of the two layers 14 and 18 comprise the same kind of oxygen ion conducting oxide (such as ZrO 2) and the same stabilizing oxide (as Y 2 O 3), but differ in the amount of the stabilizing oxide (such as 3 mol% in one layer and 5 mol% in the other) or be composed of the same conductive oxide of the oxygen ion but using different kinds of stabilizing oxide (such as Y203 for one layer and CaO for the other), as long as the two solid electrolytes are substantially similar in their coefficient of thermal expansion. It is better to form these two layers

  solid electrolyte 14 and 18 by an agglomeration process.

  simultaneous interpretation - for the same reasons as

  described above for the armor layer 12 and the first

  Accordingly, it is preferable to produce the multilayered structure of the oxygen sensing element 10 by first placing the five layers 12, 14, 16, 18 and 20 (and also the protective layer 22 if it is appropriate) one above the other in an uncooked state (not yet

  agglomerate) and then agglomerate all the layers

  at the same time by a cooking process.

  Figure 2 illustrates the application of a detection element

  both the oxygen 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 detecting the

  oxygen concentration in the exhaust gas.

  The fundamentals of this air / fuel ratio detector are disclosed in US Pat. Nos. 4,207,159 and 4,224,113, but the oxygen sensing elements of these US patents do not include the solid electrolyte layer corresponding to the first layer. electrolyte

  solid 14 according to the present invention.

  In the oxygen sensing element 10 of FIG. 2, the second solid electrolyte layer 18 and the two electrode layers 16 and 20 have a microscopically porous structure permeable to the gas molecules, while the shielding layer 12 The first solid electrolyte layer 14 may be either porous or non-porous. In this device, a source 28 of direct current is connected to the inner and outer electrode layers 16 and 20 of the element 10, in parallel with the voltage measuring device 26 to force a continuous and constant current of a determined intensity. suitably (such as about 10 pA) to flow into the second solid electrolyte layer 18 between the two electrode layers 16 and 20 to thereby cause an appropriate rate of migration of oxygen ions through this electrolyte layer. solid 18, from one of the two electrode layers 16, 20

2402187.

  to the other 20, 16. As a result of the joint effect of the migration of oxygen ions and the diffusion of the molecules

  of oxygen through the micropores of the

  In a solid state 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, if the motor is operating with a lean mixture having an air / fuel ratio greater than the stoichiometric ratio, the direct current is forced to flow into the solid electrolyte layer 18 of the outer electrode layer 20 to the inner electrode layer 16 to thereby establish and maintain a relatively low oxygen partial pressure at the interface

  above mentioned in element 10.

  The existence of the first solid electrolyte layer 14 does not affect the function of the oxygen concentration cell constituted by the second electrolyte layer

  solid 18 and two electrode layers 16 and 20.

  As described in US Pat. No. 4,224,113, the device of FIG. 2 makes it possible to detect real values of the air / fuel ratio of a mixture supplied to the engine, without having to use any additional source for a partial pressure of oxygen of

reference.

  Figure 3 shows an element detecting oxygen

  Also according to one embodiment of the invention.

  This element 10A is basically similar to the element of FIG. 1 and, as the only construction modification, there is used a grid-like internal electrode layer 16A having a number of openings 17 in place of the forming layer. internal electrode 16 formed uniformly of the element 10 of FIG. 1. The grid-shaped electrode layer 16A is employed to allow the two solid electrolyte layers 14 and 18 to come into contact with each other , not only by the marginal regions 14a

2472187.

  but also through the openings 17 of the electrode layer 16A, by filling the openings 17 with the solid electrolyte during the formation of the second

  solid electrolyte layer 18 (or first layer 14).

  As a natural consequence, the inner electrode layer 16A is even more firmly held in fixed position by the two solid electrolyte layers 14, 18, which further lowers the possibility of separation of one of the two layers 14, 18. In producing this element 10A, it is particularly desirable to form at least the two solid electrolyte layers 14, 18 and the inner electrode layer 16:

  employing a simultaneous agglomeration process.

  Of course, the oxygen detecting element 10A of FIG. 3 can be used in the air / fuel ratio detection device shown in FIG.

figure 2.

  FIG. 4 shows another modification of the oxygen detecting element 10A of FIG. 3. Indeed, the oxygen detecting element 10B of FIG. 4 comprises a heating element 30 which is either a thin wire or a thin film of an electrically resistive metal embedded in the shielding layer 12. In 32 are indicated conductors for supplying a heating current to the heating element 30. In other respects, this element 10B is identical to the 10A of Figure 3. As illustrated, the oxygen sensing element 1OB can also be used and is quite suitable for the air / fuel ratio sensing device described with reference to FIG. The solid conducting electrolyte of the oxygen ion has an inherent property that, at relatively low temperatures, the conductivity of the oxygen ions in the second solid electrolyte layer 18 becomes so low that the ment oxygen sensing may not function well. Therefore, it is necessary to provide an oxygen detecting element according to the invention with a heating means if it is desired to use this element even in gas atmospheres at relatively low temperature.For example in the case of the detection of air-fuel ratio of an internal combustion engine even during its start-up time when the temperature of the exhaust gas is not high enough. In such a case, the heater element 30 will be designed to maintain the second solid electrolyte layer 18 at a temperature of the order of 500-700WC. Of course, the heater 30 illustrated in FIG. 4 is not limited to the element 10A of FIG. 3 but can also be

  provided on the element 10 of Figure 1.

  FIG. 5 shows an element detecting oxygen 10C,

  also according to one embodiment, which is fundamentally

  similar to item 10A in Figure 3 with the excep-

  that the outer electrode layer 20 formed uniformly in FIG. 3 is replaced by an outer electrode layer 20A in the form of a gate having a number of openings 21 filled with the material

  of the protective layer 22. With this amendment,

  cation, the outer electrode layer 20A is more securely held in a fixed position and accordingly,

  it can less separate from the second layer of electro-

  This element 10C can also be applied to the air / fuel ratio detector shown in FIGS. 2 and 4. It will be understood that the outer grid-like electrode layer 20A of FIG. element 10 of Figure 1 and / or it may be used together with

the heater 30 of FIG.

  The shielding layer 12 in an oxygen sensing element according to the invention is usually made of a ceramic such as alumina, mullite, spinel, forsterite, or steatite, and it is also possible to use a cermet, that is to say a bonded or bonded mixture of a ceramic material and a metal. The shielding layer 12 for serving as a substrate for the oxygen-sensing element is produced, for example, by agglomerating a green sheet prepared by molding or extruding a wet composition containing a pulverulent raw material. for a selected ceramic, as the main component, by agglomerating a press-formed powder material or by machining an agglomerated plate into a selected ceramic. In the case where the second solid electrolyte layer 18 can serve as a substrate, the shielding layer 12 may be formed into a relatively film-like layer, e.g., by a physical deposition technique such as a spray, by plasma spraying or by the steps of printing a paste containing a powdery ceramics first material on the substrate and then agglomerate

the printed layer of pate.

  The material for each solid electrolyte layer 14, 18 may be selected from oxygen ion conductive solid electrolyte materials for conventional oxygen sensors of the concentration cell type. As examples, ZrO 2 stabilized with CaO, Y 2 O 3, SrO, MgO, ThO 2, WO 3 or Ta 2 O 3; Bi20O stabilized with Nb205, SrO, W03, Ta205 or Y203; and Y 203 stabilized with ThO2 or CaO. The importance of the essential identity between the materials of the first and second solid electrolyte layers 14 and 18 in the present invention is as previously described. If the shielding layer 12 is to serve as a substrate, each solid electrolyte layer 14, 18 may be in the form of a thin film layer formed by a physical deposition technique such as a spray or ion plating or by a method comprising the steps of printing a paste containing a solid solid electrolyte material onto the substrate 12 or the inner electrode layer and then firing the substrate where the paste is applied. In the case where the second solid electrolyte layer 18 is to serve as a substrate, this

  layer 18 can be produced, for example, by agglomerate.

  pulverulent material formed on the press or agglomeration of a green sheet containing a

  powdery solid electrolyte material.

  The layers forming internal and external electrodes 16

  (16A), 20 (20A) are made of an electronic material-

  A conductive material selected from electrode materials known to be useful for conventional solid electrolyte oxygen sensors. Examples of platinum group metals which catalyze the oxidation reactions of hydrocarbons, carbon monoxide and the like, such as Pt, Pd, Ir, Ru, Rh and Os, including the alloys of these metals, are described. platinum group and platinum group metal alloys with a base metal; and some other metallic materials like

  Au, Ag and SiC that do not catalyze oxidation reactions

  above. It is also possible to use a cermet containing one of the above metals and a ceramic, preferably an oxide serving as the main component of the solid electrolyte layers 14, 18 in the same oxygen-detecting element, in particular for the inner electrode 16 (16A) to obtain a further improved adhesion strength of this cermet electrode layer with the solid electrolyte layers 14, 18. For an oxygen sensing element for use as a shown in Fig. 1, preferably the inner electrode layer 16 (16A) is formed of an electronically conductive mixture of a metal and its oxide, such as Ni-NiO, Co-CoO or Cr-Cr203, which serves as the source the reference oxygen partial pressure on this layer forming

  electrode 16 (16A) in this element detecting oxygen 10.

  Each electrode layer 16 (16A), (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 electrochemical technique. represented by plating or printing a paste containing an electrode powder material followed by firing the printed paste layer. For the protective layer 22, it is possible to use a ceramic such as alumina, spinel, mullite or calcium zirconate. The porous protective layer 22 may be produced, for example, by plasma spraying or spraying, by spraying, by printing a paste containing a selected ceramic raw material (or applying the dough by a different technique), and subsequent baking. the printed paste layer, or by immersing the oxygen sensing element in a slurry of a material

  chosen powder, to dry the porridge adhering to the

  then cook the item thus treated.

  Typical electrically resistive metals to be used for the heater element 30, Pt, W and Mo. The flooding of the heater element 30 in the shielding layer 12 can be obtained, for example, by producing the shielding 12 by tacking two sheets face-to-face with the interposition of a metal wire or by forming, beforehand, a thin layer of metal on one of the two sheets by printing a metal paste

on this sheet.

Example 1

  Figs. 6A to 6G illustrate a method employed in this example to produce an oxygen sensing element 40 having basically the construction shown in Figs. 1 and 2, and designed to serve as an element of an air / fuel ratio detector. of the type illustrated on

Figure 2.

  Referring to FIGS. 6A-6C, a green sheet> 0 of alumina 41 (formed of a wet alumina-based composition, 5 x 9 mm wide and 0.7 mm thick), and another green alumina leaf 43 similar. by its material and its dimensions, at the first sheet 41, but crossed by two orifices 43a and 43b (0.6 mm diameter) otet used to form a shielding layer 42, to be fired at a later stage to become an organ of basic structure of the element detecting oxygen 4-0! As can be seen in FIG. 6A, two 0.2 mm platinum wires were partially placed on the alumina sheet 41. Then, the pierced sheet 43 was placed on the first sheet 41 so that the ends of the two son 45 are placed just below the two orifices 43a and 43b respectively, as can be seen in Figure 6C, and the two sheets 41, 43 are adhered to each other in this state, by application a pressure of the order of 9.81 bar to give a shielding layer 42, having already been provided with sononductor: 5 but without

to be cooked again.

  A solid electrolyte paste was prepared by uniformly dispersing 70 parts by weight of powdery ZrO 2 -Y 2 O 3 (95: 5 molar ratio) in 30 parts by weight of a lacquer comprising a resin binder and an organic solvent. The paste was applied to an outer surface (the outer surface of the pierced sheet 43) of the uncured shielding layer 42 by a screen printing technique to form a layer of dough

  as shown in Figure 6C by the hatched area.

  The drying of this layer of dough gave a layer

  uncured solid electrolyte 44, having 10-12 j of thickness.

  Then, a platinum paste, dispersion of 70 parts by weight of platinum powder in 30 parts by weight of a lacquer, was printed on the outer surface of the uncured solid electrolyte layer 44 in the pattern indicated on Figure 6D by the hatched area. A marginal region 44a of the solid electrolyte layer 44 was left uncovered substantially all the way around the resulting platinum paste layer, although this layer of paste was forced to expand locally to the surface. one of the two apertures 43a of the shielding layer 42 to fill this hole 43a with the pin of the plate. This layer of platinum was dried to obtain an internal electrode layer 46, which was:

in this state, 6-7 days thick.

  Referring to Fig. 6E, the above solid electrolyte pellet was printed on the outer surface of the uncured inner electrode layer 46 to cover the marginal region 44a of the first solid electrolyte layer L4 also by means of this screen as indicated by the hatched area. The secretion of the layer of e thus formed gave a layer of solid electrolyte which had not yet been fired and which, in this state, had a thickness of 20-22 m. As a result, the uncured internal electrode layer 46 was substantially completely (except for the elongate portion -0 extending to the orifice 43a: this portion may be considered part of a driver) enclosed

  by the two solid electrolyte layers 44 and 48.

  Then, the platinum? A above mentioned

  was printed on the outer surface of the

  uncured solid lime-4C in the pattern shown in Fig. 6F by the shaded area, and then dried to form an uncured outer electrode layer 50 and in this state having a thickness of 6-7 p. At this stage, the platinum paste layer D0 extended locally up to the orifice 43b of the shielding layer 42 to

  fill this orifice l3b with the platinum paste.

  Then, the five-layer article in the state of FIG. 6F was totally subjected to a firing process carried out in atmospheric air for 2 hours at a temperature of 1500 ° C. in order to obtain a simultaneous agglomeration of the five layers. 42, 44, 46, 48 and 50. By this firing process, the shielding layer 42 became a rigid ceramic (alumina) plate having a tight, gas-impermeable structure, while each of:, o two layers of solid electrolyte 44, 48 and two electrode layers 46, 50 became microscopically

  porous and permeable to gas molecules.

  Referring to Figure 6G, the production of

  the oxygen-detecting element 40 was completed by spraying

  or plasma spray of a zirconia powder.

  te-calcium (CaO-Zro2) on the outer front surfaces of the fired member of Figure 6F to form a porous and gas permeable protective layer 52 of 80 to 100 f thickness. A practically usable sensor for an air / fuel ratio sensor, such as that illustrated in FIG. 7, was manufactured using the oxygen sensing element 40 of FIG. 6G. Element 40 was mounted on

  an aluminum rod 56 using a ceramic cement.

  The rod 56 was formed with two axial bores through which the conductive wires 45 of the element 40 extended outwardly. The alumina rod 56 fitted very precisely into a tubular support 58 of stainless steel, and a stainless steel hood 60 having openings 61 was welded to the front end of the holder 58 to enclose the sensor element. oxygen 40. A stainless steel tube 64 was welded to the rear end of the support 58 and a nut 66 for fixing the support 58, for example, to the exhaust pipe of an automobile, was fixed around this tube 64 and welded to the rear end of the support 58 and to the outer surface of the tube 64. The lead wires 45 passed through this tube 64, and a leading end portion of this tube 64 was filled with a synthetic rubber sealant 68 the remaining part being filled with an insulator

  ceramic 69, essentially alumina powder.

  This air / fuel ratio detector was subjected to

  evaluation test as will be described below.

Example 2

  Referring to FIGS. 8A-8C, there was produced.

  in this example, an oxygen sensing element 40A having basically the construction shown in FIG. 3 and designed to serve as an element of a ratio detector

  air / fuel of the type shown in Figure 2.

  The uncured alumina shielding layer 42 represents

  felt on the figure-.8A was. prepared by assembling the two green alumina leaves 41, 43 and two platinum wires described in Example 1 by the same process as in Example 1, and an uncured solid electrolyte layer 44 was formed on the uncovered shielding layer 42 using the solid electrolyte paste described in the example

  1 to have a thickness of 10-12 p in the dried state.

  Then, the platinum paste described in Example 1 was applied to the outer surface of the layer

  uncured solid electrolyte 44 by a printing technique.

  on the screen to form a dough layer in a grid pattern as shown in Fig. 8A by the hatched area. This layer of paste was forced to extend locally to the orifice 1-a of the shielding layer 42 in order to fill this orifice 43a of the platinum paste. In this printing process, a marginal region 44a of the solid electrolyte layer 44 was left uncovered. The drying of the platinum paste layer gave a grid-like internal electrode layer 46A having a multiplicity of openings 47. In the dried and uncured state, this layer 46A had

6-7 p thick.

  Referring to FIG. 8B, another solid electrolyte layer 48 was formed on the uncured electrode layer 46A by printing the above-mentioned solid electrolyte paste to cover the outer surface of the forming layer. electrode 46A and the marginal region 44A of the first solid electrolyte layer 44 and at the same time fill the apertures 47 of the grid-like electrode layer 6A, thereby drying the resulting pdate layer. In the dry and uncooked state,

  this solid electrolyte layer 48 had 20-22 wt.

  sor. Then, the platinum pa above was printed on the outer surface of the uncured solid electrolyte layer 40 as shown in FIG. 8B by the shaded area 50. In this printing process, the

  layer of pr. Platinum 50 was forced to expand

  1 to the port 45b in the shielding layer 42> 35 to fill this port 4Db of the platinum p-te. After drying, this layer of pt +, e, i.e. uncured outer electrode layer 50 was 6-7 μm thick. Then, an alumina paste prepared by dispersing parts by weight of alumina powder, having been used for the preparation of the shielding layer 42, in 30 parts by weight of lacquer printed on the outer surfaces of the front side of the multilayered article of Figure 8C in a pattern shown in Figure 8C by the hatched area, and then dried to form a

  protective layer 52A, uncured.

  The oxygen detecting element 40A was terminated by subjecting the six layer article in the state of FIG. 8C to a quenching process which was carried out in air for 2 hours at a temperature of 15,000 C to obtain the simultaneous agglomeration of six layers 42, 44, 46A, 48, and 52A. By this firing process, the shielding layer 42 became a rigid ceramic (alumina) plate having a tight, gas-impermeable structure, while each of the two layers. solid electrolyte 44, 48 and two electrode layers 46A, 50 and the protective layer 52A became microscopically

Porous and permeable to gases.

  An air / fuel ratio detector having the construction shown in Fig. 7 was also manufactured using the oxygen sensing element 40A

of Figure 8C.

Reference 1

  For the comparison in the evaluation test below

  mentioned above, an oxygen detecting element not in accordance with the present invention has been produced by omitting the formation of the first solid electrolyte layer 14

  of the method employed in Example 1.

  Referring to Figure 9A, the platinum paste used in Examples 1 and 2 was printed on the surface of uncured shielding layer 42 that had been prepared by the process described in Example 1, to form a layer of ipte according to the pattern indicated by the shaded area, with an extension to the hole 43a of the shielding layer 42 to fill the hole 43a of the dough. This mud layer was dried to give an inner electrode layer 76 not yet fired and having, in this state, 6-7 μm thick. Then, a solid insulation layer 78 shown in Fig. 9B was formed to cover the uncured electrode layer 76 substantially completely, and also the outer surface of the shielding layer 42 in an area surrounding the periphery of the layer. electrode 76, employing a solid electrolyte lye and the printing method used in Example 1 to thereby form the second solid electrolyte layer 78. In the dry but uncured state, this

  Solid electrolyte layer 78 was 20-22 μm thick.

  Next, an outer electrode layer 80 as shown in Fig. 9C was formed on uncured solid electrolyte layer 78 using the platinum pBe above and the process described for Fig. 6F of Example 1. In the dry but uncooked state,

  this layer 80 was 6-7 f thick.

  Next, the four-layer article in the state of FIG. 9C was subjected to the same cooking process as that performed in example 1 (1500 C, 2 hours) to obtain a simultaneous agglomeration of the four layers 42, 76, 78 and 80. As a result, the armor layer 42 became

  a rigid ceramic plate having an impermeable structure

  while the solid electrolyte layer 78 and each of the electrode layers 76 and 80

  became microscopically porous and gas permeable.

  The production of a detecting element: the oxygen 70 shown in FIG. 9D has been completed by forming a porous protective layer 82, identical to the protective layer 52 formed in example 1 by the material (zirconate-calcium), the formation process (spraying or plasma spraying) and thickness (80-100)

Reference 2.

  Also for comparison, an oxygen sensing element not related to the invention was produced, omitting the formation of the first electrolyte layer.

  solid 44 of the method employed in Example 2.

  Referring to FIG. 10A, a gate-shaped inner electrode layer 76A was formed on uncured shielding layer 42 prepared according to Example 1 by the electrode-forming process described in FIG. Referring to Figure 9A with the exception of changing the printing pattern. In the dry but uncured state, the electrode layer 76A was 6-7μ thick. Then, a solid electrolyte layer 78 as can be seen in FIG. 10B, was formed by the process described with reference to FIG. 9B, taking care to fill the openings 77 of the uncured electrode layer 76A by means of solid electrolyte prays. In the dry but uncured state, this solid electrolyte layer 78 had

20-22 p thick.

  Next, forming an outer electrode layer corresponding to layer 80 of FIG. 9C, the firing process to obtain agglomeration

  simultaneously the shielding layer, the

  solid layer and two layers of electrodes and the formation of a porous protective layer were performed sequentially exactly according to the steps of the reference

  nbl described with reference to Figures 9C and 9D.

  Evaluation Test The oxygen-sensing elements of Examples 1 and 2 and references Nos. 1 and 2 were tested in the form of the air / fuel ratio detector of Figure 7. For each of these four kinds of sensing elements, oxygen, five samples were subjected to the same test, the test was an endurance test. A motor gasoline engine was mounted on a bench test rig, and each sample of the air / fuel ratio detectors in Figure 7 was attached to the pipe.

  engine exhaust to allow exhaust gases to

  flow and pass through the interior of the open hood 60 of the detector. The engine was powered by an air-fuel mixture whose air / fuel ratio was

24721 87

  somewhat weaker than the stoichiometric ratio, and operated in plei1úgaz condition. Because of

  the use of a fuel-rich mixture, exhaust gases

They were at a very low oxygen concentration and contained about 5% carbon monoxide. The exhaust gas temperature at the location of the air / fuel ratio sensor was consistently in the range of 8300C. A continuous and constant voltage source was connected to the air / fuel ratio sensor coils 45 to force a constant current of a predetermined intensity to flow into the solid electrolyte layer between the two electrode layers of the sensing element. oxygen, from the inner electrode layer to 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 electrode layer

  solid lyte, and a voltmeter was connected between the two conductors 45 to measure the developed output voltage

  by the oxygen detecting element exposed to the exhaust gases.

  ment. During an initial stage of the endurance test, each sample of the air / fuel ratio sensor produced a stable output voltage accurately indicating the air / fuel ratio of the mixture supplied to the engine. However, at first the output of the samples comprising the oxygen detecting element 70 produced at the reference n01 became unstable, and after having subsequently tested the endurance test for 50 hours, it was found that a separation had occurred. between the inner electrode layer 76 and the shielding layer 42 in each of the five samples of this type of oxygen detecting element 70. The samples comprising the oxygen detecting element of the reference n02 exhibited a high endurance, but after continuation of the endurance test for 200 hours, it was confirmed that the

  separation had occurred between the

  internal trode (grid-shaped) 76A and the

  shield 42 in each of the five samples.

  In contrast, all samples including the oxygen-sensing element 40 of Example 1 or the element 40A of Example 2 continued to exhibit a stable function even after 200 hours from the start of the test. endurance. It was confirmed that the inner electrode layer 46 or 46A of each sample remained in close contact with the inner and outer solid electrolyte layers 44 and 48 without appreciable separation therebetween. For samples comprising element 40A of Example 2, there was no separation between the grid-shaped inner electrode layer 46A and each of the two solid electrolyte layers 44 and 48 even during an examination. after continuation of

  the endurance test for 250 hours.

  Of course, the invention is in no way limited to the embodiments described and shown which have been given by way of example only. In particular, it includes all the means constituting technical equivalents

  described means as well as their combinations if these

  these are executed according to his spirit and implemented in

  the framework of protection as claimed.

Claims (14)

R E V E N D I C A T I 0 N S ___________________________   An oxygen-oxygen-type oxygen-detecting element using a solid electrolyte conducting the oxygen ion, characterized in that it comprises: a shielding layer (12) of a ceramic material; a first layer of an oxygen ion conductive solid electrolyte (14) disposed on and in close contact with a major surface of said shielding layer; an inner electrode layer (16) disposed on and in close contact with the outer surface of said first solid electrolyte layer; a second layer of an oxygen ion conductive solid electrolyte (18) disposed on the surface of said first solid electrolyte layer (14) to substantially closely cover said inner electrode layer (16); and   an outer electrode layer (20) disposed   on and in close contact with the outer surface of said second solid electrolyte layer (18); said second solid electrolyte layer (18) and said inner and outer electrode layers (16, 20) constituting an oxygen concentration cell, the material of said first layer (14) being at least substantially the same as that of said second layer (18).   Element according to claim 1, characterized in that said inner electrode layer (16) is formed so that a marginal region (14a) of said first solid electrolyte layer (14) is left uncovered substantially. all around said layer (16), the second solid electrolyte layer (18) being formed so that its marginal region is in close contact with said region   marginal (14a) of said first layer.   3. Element according to claim 2, characterized in that the material of the second solid electrolyte layer (18) above is composed of a major amount of an oxide of a metal conductive oxygen ion as the main component and a minor amount of at least one stabilizing oxide, the material of the aforementioned first solid electrolyte layer (14) containing a major amount of said oxygen ion conductive metal oxide as a main component and a amount   minor of said stabilizing oxide.   4. Element according to claim 3, characterized in that the material of the first solid electrolyte layer (14) above is the same as that of the second solid electrolyte layer (18) above also for the proportion of oxide of a conductive metal of the ion   oxygen to the stabilizing oxide.   5. Element according to claim 3, characterized in that the material of the first solid electrolyte layer (14) above is different from that of the second solid electrolyte layer (18) mentioned above by the proportion of the solid oxide. Oxide conductive metal with stabilizing oxide, 6. Element according to claim 1, characterized in that at least one of the inner and outer electrode layers is a grid-shaped layer (16A). 20A) having a number of openings (17, 21), said openings being filled with the material of a layer (14, 18,   22) adjacent to said grid-shaped layer (16A, 20A).   Element according to any one of the claims   1, 2 or 3, characterized in that the above-mentioned inner electrode layer is grid-shaped (16A) having a number of openings (17), one of the first and second solid electrolyte layers (14, 18) mentioned above being formed in order to intrude and filling said openings (17).   18. Element according to claim 3, characterized in that at least the inner electrode layer (16) above or the outer electrode layer (20)   mentioned above is made of a cermet which is   metal and the oxide of a conductive metal the aforementioned oxygen ion.   9. Element according to claim 1, characterized in that at least the shielding layer (12) above, the first solid electrolyte layer (14) above, the inner electrode layer (16) above and the second layer of said solid electrolyte (18) is formed by the steps of disposing said layers one over the other while each of said four layers is in an uncured state and firing said layers in the multilayered state to obtain simultaneous agglomeration said four layers.   Element according to any one of the claims   1 or 2, characterized in that the above-mentioned shielding layer (12) is formed thick enough to serve as a basic structural member of said element, each of said first and second solid electrolyte layers (14, 18) and said inner and outer electrode layers (16, 20) being a thin film-like layer. 11. Element according to claim 1, characterized in that it further comprises an electric heater element (30) embedded in the shielding layer (12) above.   Element according to any one of the claims   1 or 11, characterized in that it further comprises a porous and gas permeable protective layer (22) formed of a ceramic and which covers at least the outer surfaces of the above-mentioned outer electrode layer (20) and the second solid electrolyte layer (18) above.   Element according to any one of the claims   1 or 2, characterized in that the second layer of electro-   said solid layer (18) and said inner and outer electrode layers (16, 20) having a microscopically porous and gas-permeable structure, said shielding layer (12) having a sealed structure impervious to gases.   Element according to claim 13, characterized in that each of said inner and outer electrode layers (16, 20) is made of a metal capable of catalyzing hydrocarbon oxidation reactions. and carbon dioxide.   15. Element according to claim 1, characterized in that the inner electrode layer (16) above is made of an electronically conductive mixture of a metal and an oxide of said metal, said mixture having the function of producing a partial pressure oxygen   reference in said element.   16. Device for detecting the air / fuel ratio of an air-fuel mixture supplied to a combustion means, characterized in that it comprises a detecting element   the oxygen (10) according to any one of the claims   and a DC source (28) connected to the aforementioned inner and outer electrode layers (16, 20) to force a constant current at an intensity   predetermined through the second electrolyte layer   said solid (18) between said inner and outer electrode layers for causing migration of the oxygen ions through said second solid electrolyte layer from one of said inner and outer electrodes to the other to thereby establish a partial pressure of reference oxygen at the interface between said inner electrode layer (16) and the second solid electrolyte layer (18); and voltage measuring means (26) connected to said inner and outer electrode layers (16, 20) for measuring the output voltage   produced by said oxygen sensing element.