GB2087569A

GB2087569A - Oxygen sensor element having thin layer of stabilized zirconia sintered on substrate

Info

Publication number: GB2087569A
Application number: GB8133988A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1980-11-12
Filing date: 1981-11-11
Publication date: 1982-05-26
Also published as: FR2493990A1; JPS5782761A; GB2087569B; DE3144838A1

Abstract

In an oxygen sensor element of the concentration cell type having a thin layer (14) of an oxygen ion conductive solid electrolyte formed by sintering a powder mixture of zirconia and a stabilizing metal oxide on a plate-like substrate (10) a reference electrode layer (12) in contact with the solid electrolyte layer and a measurement electrode layer (16) formed on an outer surface of the solid electrolyte layer; the volume percentage of cubic crystals in the sintered zirconia layer (14) is from 40 to 65% to thereby improve the resistance of this layer to severe and repeated heat shocks as may be experienced in exhaust systems of automobile engines. The stabilising oxide is preferably one or more of yttria, erbia, calcia or magnesia present in a total molar percentage of 4.0 to 5.5. The crystal grain size in the solid electrolyte is preferably between 3 and 15 mu m. <IMAGE>

Description

SPECIFICATION Oxygen sensor element having thin layer of stabilized zirconia sintered on substrate This invention relates to an oxygen sensor element of the concentration cell type having a relatively thin layer of an oxygen ion conductive solid electrolyte which is sintered on and at least partly in contact with a plate-like substrate, and more particularly to an oxygen sensor element of this type in which zirconia containing a stabilizing metal oxide is used as the solid electrolyte.

The usefulness of oxygen sensors of the concentration cell type that utilizes an oxygen ion conductive solid electrolyte typified by zirconia (ZrO2) has been well appreciated in various fields. In the automobile industry it has currently become popular to provide an oxygen sensor of this type in the engine exhaust system for the purpose of detecting deviations of actual air/fuel ratio of a gas mixture supplied to the engine from an intended value based on the amount of oxygen contained in the exhaust gases. The oxygen-sensitive part of the sensor has a sintered solid electrolyte layer, a measurement electrode layer formed on a surface of the solid electrolyte layer so as to be exposed to a gas subject to measurement and a reference electrode layer formed on the solid electrolyte layer in an area 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 the 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 20 microns and each of the two electrode layers is made still thinner. The concentration cell of the laminated structure is physically supported by a thin plate of a ceramic material such as alumina, which plate is called a substrate and may be only about several millimeters in length and width and less than 1 mm in thickness. In some cases, the solid electrolyte layer is in direct contact with a major surface of the substrate over the entire area of this layer.In other cases, the reference electrode layer is formed directly on the surface of the substrate, and the solid electrolyte layer is formed so as to substantially entirely cover the reference electrode layer and make direct contact with the substrate surface in a marginal region of the solid electrolyte layer.

In practice, zirconia is predominantly used as the solid electrolyte material in this type of oxygen sensors because of its excellent physical and electrochemical properties and relatively low price. As to the method of forming the solid electrolyte layer on the substrate, it is usual to employ a printing-firing method including the steps of applying a paste containing a zirconia powder onto the substrate by screen-printing to form a paste layer of the desired pattern, drying the printed paste layer and firing the dried layer together with the substrate to achieve sinterning of the zirconia powder contained in the applied paste.

As is known, the crystal form of pure zirconia depends on temperature. In the temperature range of about 900-1 2000C there occurs a reversible transformation from the monoclinic form to the cubic form, and a change of about 79% in the volume of the zirconia mass accompanies this transformation. Therefore, it is impractical to use pure zirconia as the material of the solid electrolyte layer of the above described oxygen sensor as the oxygen sensor is subjected to great and repeated changes in the exhaust gas temperature, significant changes in the volume of the zirconia layer will result in cracking of the layer or peeling of the layer from the substrate.Accordingly it is usual to add a small amount of a stabilizing metal oxide such as calcia (CaO) or yttria (Y203) which is effective for forming a thermally stable solid solution or compound of the cubic system to thereby minimize variations in the volume of the sintered solid electrolyte layer with variations in the temperature.

With respect to rather primitive and relatively large-sized oxygen sensors having a tube-shaped zirconia body, there is some literature relating to the manner of stabilizing zirconia. For example, Japanese Patent Application Primary Publication No. 53(1978)-i 28612 proposes the use of a relatively small amount of stabilizing oxide so as to result in the coexistence of 5 to 35% by weight of the monoclinic crystals in the zirconia body, while Japanese Patent Application Primary Publication No.

54(1979)34309 proposes the use of such an amount of stabilizing oxide that the proportion of the cubic crystals in the circonia body reaches 93 to 99%. With respect to the miniaturized oxygen sensors of the above described lamination type, however, our experiments have revealed that neither of the proposals of these Japanese patent applications is suitable for producing a thin solid electrolyte layer which is sufficiently resistant to severe heat shocks and exhibits a long service life in automobile exhaust systems without cracking or peeling off the substrate. Because of extremely small heat capacity of the miniaturized oxygen sensor, it will be necessary to more deeply and cautiously consider the crystal form and physical properties of the solid electrolyte layer.

It is an object of the present invention to provide an improved oxygen sensor element of the concentration cell type having a relatively thin layer of stabilized zirconia which is sintered on and at least partly in contact with a plate-like substrate, the thin zirconia layer of which sensor element has high resistance to heat shocks and accordingly is particularly suitable for use in exhaust systems of automobile engines.

An oxygen sensor element of the concentration cell type according to the invention has a platelike substrate of a ceramic material, a thin layer of an oxygen ion conductive solid electrolyte which is formed by sintering a powder mixture of a major amount of zirconia and a minor amount of at least one stabilizing metal oxide on the substrate and is either partly or entirely in close contact with a major surface of the substrate, a reference electrode layer a major surface of which is in close contact with the solid electrolyte layer, and a measurement electrode layer which is formed on an outer surface of the solid electrolyte layer and is spaced from the reference electrode layer.The improvement according to the invention resides in that the proportion of the cubic crystals of zirconia in the solid electrolyte layer at room temperature as determined by X-ray diffraction analysis is in the range from 40.0 to 65.0%. In the present application, the proportion of the cubic crystals in the sintered solid electrolyte layer is given by

wherein lCz represents the integrated intensity of the peak of the diffracted beams characteristic of the (111) planes of cubic crystals of zirconia, IM(111) represents the integrated intensity of the peak of the diffracted beams characteristic of the (111) planes of monoclinic crystals of zirconia, and M(111) represents the integrated intensity of the peak of the diffracted beams characteristic of the (111) planes of monoclinic crystals of zirconia.

For monoclinic crystals of zirconia, the peak of the diffracted beams characteristic of the (111) planes and the peak of the diffracted bearris characteristic of the (11 T) planes are X-ray crystallographically paired. Accordingly, the volume percentage of cubic zirconia in the solid electrolyte layer is accurately determined by the ratio of lC(1lil to (IM(111) + ism(111) + ciii)' The stabilizing metal oxide(s) can be selected from the conventionally used metal oxides, but it is particularly preferred to use yttria (Y203), erbia (Er203), calcia (CaO) or magnesia (MgO).The realize a desired percentage of cubic crystals of zirconia as specified above, it is suitable that the molar ratio of the stabilizing metal oxide(s) to zirconia falls in the range from 4.0:96.0 to 5.5:94.5.

The solid electrolyte layer according to the invention is highly resistant to severe and repeated heat shocks and rarely cracks or peels off the substrate even when subjected to rapid and repeated rises and falls of temperature between 3000C and 8000C by way of example. Therefore, an oxygen sensor element according to the invention exhibits good durability and sufficiently long service life when used in exhaust systems of automobile engines.

In the accompanying drawings: Fig. 1 is a schematic plan view of an oxygen sensor element in which the present invention is embodied; Fig. 2 is a schematic sectional view taken along the line 2-2 in Fig. 1; Fig. 3 is a schematic sectional view of a differently designed oxygen sensor element as another embodiment of the invention; and Fig. 4 is a schematic sectional view of a still differently designed oxygen sensor element also as an embodiment of the invention.

Figs. 1 and 2 show a fundamental construction of an oxygen sensor element embodying the present invention. A structurally basic member of this element is a base plate or substrate 10 which is made of a ceramic material such as alumina, mullite orforsterite by way of example.

A reference electrode layer 1 2 is formed on a major surface of the substrate 10 in a pattern as shown in Fig. 1. An elongately extending part 1 2a of this electrode 12 is for connection with a lead wire (not shown). A layer 14 of stabilized zirconia as an oxygen ion conductive solid electrolyte is formed on the same side of the substrate 10 so as to closely and substantially entirely cover the reference electrode layer 12 and make direct contact with the surface of the substrate 10 in a marginal region surrounding the electrode layer 12. A measurement electrode layer 1 6 is formed on the outer surface of the solid electrolyte layer 14. An elongately extending part 1 6a of this electrode layer 1 6 is for connection with a lead wire (not shown).The solid electrolyte layer 14 is a thin, film-like layer (though regarded as a "thick film" in the field of current electronic technology) usually having a thickness of 10 to 30 zIm. The two electrode layers 12, 1 6 are still thinner and may be about 5 Hm, respectively.

These three layers 12, 14, 1 6 constitute an oxygen concentration cell, and for operation of this sensor element there is the need of establishing a reference oxygen partial pressure at the interface between the reference electrode layer 12 and the solid electrolyte layer 14. According to U.S. Patents Nos. 4,207,1 59 and 4,224,113, such a reference oxygen partial pressure is established by supplying a DC current to this concentration cell such that a current of an adequate intensity flows through the solid electrolyte layer 14 to keep oxygen ions migrating through this layer 14 between the reference electrode layer 1 2 and the measurement electrode layer 1 6 in a selected direction at an adequate rate.

As a joint effect of the migration of oxygen ions and diffusion of oxygen molecules through the solid electrolyte layer, a constant oxygen partial pressure is maintained at the aforementioned interface. In the case of using this method, a preferable material of the reference electrode 12 is a metal of the platinum group or its alloy. Another method of establishing a reference oxygen partial pressure in this concentration cell is to use an electronically conducting mixture of a metal and its oxide, such as Ni-NiO, Co-CoO or Cr-Cr2O3, which serves as the source of a suitable amount of oxygen as the material of the reference electrode layer 12. In most cases, a metal of the platinum group or its alloy is used as the material of the measurement electrode layer 16.

The solid electrolyte layer 14 is formed by this aforementioned printing-firing method. Each of the electrode layers 12, 1 6, too, can be formed by a similar method by using a conductive paste containing a powdered electrode material but may alternatively be formed by a physical vapor deposition technique such as sputtering or vacuum evaporation. The present invention does not place any particular restrictions on the known materials and methods for the formation of the substrate 10 and the electrode layers 1 2 and 1 6. In practice, the concentration cell part of the oxygen sensor element may be coated with a porous protective layer (not shown) formed of a ceramic material.

According to the invention, the sintered zirconia layer 14 is made to contain such an amount of a stabilizing metal oxide that the volume percentage of the cubic crystals of zirconia in the sintered zirconia layer falls within the range from 40.0 to 65.0% when calculated in the above specified way. The resistance of the zirconia layer 14 to heat shocks lowers when the proportion of the cubic crystals is either below 40% or above 65%. Among various metal oxides known as to serve for stabilizing zirconia, preferred examples are yttria (Y203), erbia (Er203), calcia (CaO) and magnesia (MgO). If desired, it is possible to jointly use two or more of these metal oxides.

The percentage of the cubic crystals in the sintered zirconia layer 14 depends primarily on the ratio of the stabilizing metal oxide to zirconia in the raw material for this layer 14. In general, it is suitable that the molar ratio of the stabilizing metal oxide (in total when two or more kinds of metal oxides are used jointly) to zirconia falls within the range from 4.0:96.0 to 5.5:94.5.

A solid electrolyte paste for forming the solid electrolyte layer 1 4 is prepared in a known manner by uniformly dispersing a mixture of a zirconia powder and a suitable amount of a stabilizing metal oxide powder in a liquid vehicle, which is usually a solution of an organic polymeric substance in an organic solvent. A preferred example of a suitable organic polymer is a cellulose derivative such as methyl cellulose or ethyl cellulose, and an example of a solvent suitable for a cellulose derivative is terpineol.

The crystal grain size of the sintered zirconia layer 14 depends on the particle size of the zirconia powder contained in the solid electrolyte paste, and the percentage of the cubic crystals in the sintered zirconia depends on the crystal grain size and the firing or sintering temperature, too. To realize a desirable percentage of the cubic crystals, generally it is suitable to adjust the particle size of the zirconia powder such that the maximum value of the crystal grain size after sintering is not larger than 1 5 ym but is not smaller than about 3 ,um and to perform the sintering process at a temperature in the range from about 1 350CC to about 1 5500C.

The construction of an oxygen sensor element according to the invention is not limited to the illustration in Figs. 1 and 2. Fig. 3 shows another example, wherein a solid electrolyte (stabilized zirconia) layer 24 is formed directly on a major surface of a substrate 20, and a reference electrode layer 22 and a measurement electrode layer 26 are formed on the outer surface of the zirconia layer 24 in a spaced arrangement. To expose only the measurement electrode layer 26 to a gas subject to measurement, the reference electrode layer 22 is covered with a shield coating 28 of a ceramic material.

Fig. 4 shows a still different example. In this case, a first reference electrode layer 32A and a second reference electrode layer 32B are formed directly on a major surface of a substrate 30 with a space therebetween. A solid electrolyte (stabilized zirconia) layer 34 is formed on the same side of the substrate 30 so as to substantially entirely cover the two reference electrode layers 32A and 32B and make direct contact with the surface of the substrate 30 in both central and marginal regions of the solid electrolyte layer 34. On the outer surface of the solid electrolyte layer 34, there are a first measurement electrode layer 36A in a region directly above the first reference electrode layer 32A and a second measurement electrode layer 36B in a region directly above the second reference electrode layer 32B. As will be understood, this oxygen sensor element has two sets of concentration cells integrated on a single substrate.

The manufacture of an oxygen sensor element according to the invention will be illustrated by the following examples, including some experiments.

EXAMPLE 1 This example (and also the subsequent examples) relates to the production of an oxygen sensor element of the type as shown in Figs. 1 and 2.

An alumina paste was prepared by well mixing 76 parts by weight of an alumina powder having a mean particle size of about 0;31m, 9'parts by weight of polyvinyl butyral, 1 5 parts by weiqht of dibutyl phthalate and a suitable amount of ethanol as solvent. This paste was extruded to form a socalled green (unfired) alumina sheet which had a thickness of 0.7 mm, and this sheet was cut into rectangular pieces of 10 mm x 5mm width to use each of these pieces as the material of the substrate 10 of the oxygen sensor element.

A platinum paste prepared by dispersing a platinum powder having a mean particle size of 5,um in an organic liquid vehicle, which was a solution of ethyl cellulose in terpineol, was applied onto a major surface of the unfired substrate (10) by a screen-printing method in a pattern as indicated at 12 in Fig.

1, and the resultant platinum paste layer (12) was dried to evaporate the solvent contained in the applied paste.

Next, a solid electrolyte paste prepared by dispersing a powder mixture of 95 mole % of zirconia and 5 mole % of yttria in an organic liquid vehicle, which was a solution of ethyl cellulose in terpineol, was applied onto the unfired substrate (10) by a screen-printing method in a pattern as indicated at 1 4 in Fig. 1 so as to cover the principal part of the dried platinum paste layer (12), followed by drying of the resultant solid electrolyte paste layer (14).

Then, the aforementioned platinum paste was applied onto the outer surface of the dried solid electrolyte paste layer (14) by a screen-printing method in a pattern as indicated at 16 in Fig. 1 and the reusultant platinum paste layer (16) was dried.

Thereafter the laminated element was fired in air at a temperature of 1 5000C for a period of 2 hr to achieve simultaneous sintering of the substrate 10, reference electrode layer 12, solid electrolyte layer 14 and measurement electrode layer 1 6. In this case the yttria powder contained in the solid electrolyte paste has a mean particle size of 0.4 ym, whereas the particle size of the zirconia powder for the same paste was adjusted such that the maximum value of the crystal grain size in the sintered solid electrolye layer 14 was not larger than 10 ym (but not smaller than 5cm). At the stage of screenprinting of the solid electrolyte paste, the thickness of the printed paste layer was controlled such that the sintered solid electrolyte layer 14 had a thickness of 10-20 ym. By x-ray difraction analysis the percentage of the cubic crystals of zirconia in the sintered solid electrolyte layer 14 was calculated to be 56.0%.

A sufficient number of samples were produced by the same method and under the same conditions to subject them to a heat shock test described hereinafter. These samples will be referred to as Example 1A.

Another group of samples of the same oxygen sensor element were produced generally by the same procedure except that the particle size of the zirconia powder was adjusted such that the maximum value of the crystal grain size in the sintered solid electrolyte layer 14 was not larger than 14 itm (but not smaller than 10 cm). In these samples, which will be referred to as Example 1 B, the percentage of the cubic crystals of zirconia in the solid electrolyte layer was 40.0%.

As Example 1 C, another group of samples were produced generally by the same procedure except that the particle size of the zirconia powder was adjusted such that the maximum value of the crystal grain size in the sintered solid electrolyte layer 14 was not larger than 5 ym (but not smaller than 3 yam).

As a result, the percentage of the cubic system crystals in the zirconia in the solid electrolyte layer 14 was 65.0%.

A different group of samples referred to as Example 1 D were produced generally similarly to the samples of Example 1 A except that the firing process was performed at a temperature of 1 4600C for 2 hr. As the result, the percentage of the cubic crystals of zirconia in the solid electrolyte layer 14 decreased to 40.0%.

Another group of samples referred to as Example 1 E were produced generally similarly to the samples of Example 1 A except that the firing process was performed at 1 540OC for 2 hr. As the result, the percentage of the cubic zirconia in the solid electrolyte layer 14 increased to 65.0% EXAMPLE 2 The oxygen sensor element described in Example 1 was produced generally similarly to the samples of Example 1 A, but the molar proportion of the yttria powder to zirconia powder in the solid electrolyte paste was 4:96. In this example, the percentage of the cubic zirconia in the sintered solid electrolyte layer 14 was 40.0%.

EXAMPLE 3 As a sole modification of the process of producing the oxygen sensor of Example 1 A, the molar proportion of the yttria powder to zirconia powder in the solid electrolyte paste was 5.5:94.5. In this example, the percentage of the cubic zirconia in the sintered solid electrolyte layer 14 was 65.0%.

REFERENCES For the sake of comparison, several kinds of oxygen sensor element samples not in accordance with the invention were produced by modifying Example 1A in the following points, respectively.

References 1 to 4: The amount of the yttria powder in the zirconia-yttria mixture was either reduced or increased as shown in the following Table 1. This modification resulted in a variation in the percentage of the cubic crystals in the sintered zirconia, also as shown in Table 1.

References 5 to 8: the particle size of the zirconia powder was adjusted so as to make the crystal grain size in the sintered solid electrolyte layer 14 either very large or very small as shown in the following Table 2. This modification resulted in a variation in the percentage of the cubic crystals in the sintered zirconia, also as shown in Table 2.

References 9 to 12: the firing temperature was lowered or raised as shown in the following Table 3, with the result that the percentage of the crystal grain size in the sintered zirconia varied as shown in Table 3.

HEAT SHOCK TEST The oxygen sensor element samples of Examples 1-3 and References 1-12 were subjected to a heat shock test to evaluate the durability of the solid electrolyte layer 14 in each sample in hot gas atmospheres.

In this test, every sample was alternately exposed to a gas atmosphere having a temperature of 3000C and another gas atmosphere having a temperature of 8000C at constant intervals of 2 minutes for a total duration of 4000 minutes. After that, each sample was immersed in a 5% solution of Fuchsine (Magenta) in ethanol, withdrawn from the solution and washed with water. If the greenish color characteristic of Fuchsine was perceptible in the solid electrolyte layer 14 of the washed sample, the solid electrolyte layer 14 of that sample was judged to have cracked or partly peeled off the substrate 10 or the reference electrode layer 12. Tables 1 to 3 contain the results of the heat shock test, evaluating those samples which exhibited the aforementioned coloring as "NG".The samples of Reference 6 (in Table 2) were not subjected to the heat shock test because in this case it was apparent that the fired solid electrolyte layer 14 had not fully sintered.

As can be seen in Tables 1 to 3, the tested oxygen sensor elements according to the invention (Examples 1 to 3, wherein the proportion of the cubic crystals in the sintered zirconia was in the range from 40 to 6C%) were superior to the counterparts of References 1 to 1 2 in durability and resistance to heat shocks.

TABLE 1

Amount Firing Crystal Proportion Result of of Y2O3 Condition Grain of Cubic Heat Shock (mole%) Size Crystals Test ( m) (%) Ref. 1 2.0 1500 C x 2 hr # 10 0 NG Ref. 2 2.8 ,, ,, 35.0 NG Ex. 2 4.0 ,, " 40.0 OK Ex. 1A 5.0 " ,, 56.0 OK Ex. 3 5.5 ,, ,, 65.0 OK Ref. 3 6.0 ,, " 70.0 NG Ref.4 8.0 ,, " 85.0 NG TABLE 2

Amount Firing Crystal Proportion Result of of Y203 Condition Grain of Cubic Heat Shock (mole%) Size Crystals Test (,tm) (%) Ref. 5 5.0 15000Cx2hr # 53 0 - Ref. 6 5.0 ,, s 20 30.0 NG Ex.1 5.0 ,, < 14 40.0 OK Ex.1A 5.0 ,, < 10 56.0 OK Ex.1 C 5.0 ,, < 5 65.0 OK Ref.7 5.0 ,, # 3 100 NG Ref.8 5.0 " # 1 100 NG TABLE 3

Amount Firing Crystal Proportion Result of of Y203 Condition Grain of Cubic Heat Shock (mole%) Size Crystals Test (jam) (%) Ref. 9 5.0 12500Cx2hr # 10 10.0 NG Ref.10 5.0 1300 C x 2 hr ,, 20.0 NG Ref 11 5.0 1380 C x 2 hr ,, 32.0 NG Ex.1 D 5.0 1460 C x 2 hr ,, 40.0 OK Ex. 1A 5.0 1500 C x 2 hr ,, 56.0 OK Ex.1 E 5.0 1540 C x 2 hr ,, 65.0 OK Ref.12 5.0 1580 C x 2 hr ,, 80.0 NG The data presented in Table 1 represent the dependence of the proportion of the cubic crystals in the zirconia inthe sintered solid electrolyte layer 14 on the amount of yttria used as the stabilizing oxide and demonstrate the appropriateness of using 4.0 to 5.5 mole 96 of yttria.From Table 2, it can be seen that the proportion of the cubic crystals of zirconia in the sintered zirconia depends on its crystal grain size, too, and falls within the preferred range of 40-65% when the maximal value of the crystal grain size is not larger than about 1 5 jam but is not smaller than about 3 jam. The data presented in Table 3 show that the proportion of the cubic crystals in the sintered zirconia increases as the firing temperature is made higher and that a firing temperature in the range from about 1 4500C to about 1 5500C is suitable for sintering a mixture of 95 mole % of zirconia and 5 mole % of yttria.

EXAMPLE 4 The oxygen sensor element of Figs. 1 and 2 was produced generally in accordance with Example 1 A by using a powder mixture of 95 mole % of zirconia and 5 mole % of erbia as the principal material of the solid electrolyte paste to form the solid electolyte layer 14. The particle size of the zirconia powder was adjusted such that the maximum value of the crystal grain size in the sintered solid electrolyte layer 14 was not larger than 8 m (but not smaller than 5 m). Because of the use of erbia in place of yttria, the firing temperature was lowered to 1420 C. The samples produced in this manner will be referred to as Example 4A.

As Example 4B, another group of samples were produced generally similarly to the samples of Examples 4A except that the molar ratio of the erbia powder to zirconia powder was 4:96. This modification resulted in that the proportion of the cubic crystals of zirconia in the sintered zirconia decreased to 40.0%.

As Example 4C, another group of samples were produced generally similarly to Example 4A except that the molar ratio of the erbia powder to zirconia powder was 5.5:94.5. As the result, the proportion of the cubic crystals in the sintered zirconia increased to 65.0%.

The oxygen sensor element samples of Examples 4A to 4C were subjected to the above described heat shock test. The results are shown in the following Table 4.

References 13 to 1 5 For comparison, three kinds of oxygen sensor element samples not in accordance with the invention were produced generally similarly to the sample of Example 4A but by varying the mixing ratio of the erbia powder to zirconia powder as shown in Table 4. These samples were subjected to the heat shock test together with the samples of Example 4A to 4C.

TABLE 4

Amount Firing Crystal Proportion Result of of Y203 Condition Grain of Cubic Heat Shock (mole%) Size Crystals Test (jam) (%) Ref. 13 3.5 14200C x 2 hr < 8 30.0 NG Ex. 4B 4.0 ,, ,, 40.0 OK Ex. 4A 5.0 " ,, 60.0 OK Ex. 4C 5.5 ,, ,, 65.0 OK Ref. 14 6.0 ,, " 80.0 NG Ref. 1 5 8.0 ,, ,, 90.0 NG

Claims

1. An oxygen sensor element of the concentration cell type having a plate-like substrate of a ceramic material, a thin layer of an oxygen ion conductive solid electrolyte formed by sintering a powder mixture of a major amount of zirconia and a minor amount of at least one stabilizing metal oxide on said substrate and which is at least partly in close contact with a major surface of said substrate, a reference electrode layer a major surface of which is in close with said solid electrolyte layer and a measurement electrode layer which is formed on an outer surface of the solid electrolyte layer and is spaced from SAID reference electrode layer, characterized in that the proportion of the cubic crystals of zirconia in said solid electrolyte layer at room temperature determined by X-ray diffraction analysis (as hereinbefore defined) is in the range from 40.0 to 65.0%.

2. An oxygen sensor according to Claim 1, wherein the stabilizing metal oxide is yttria, erbia, calcia or magnesia.

3. An oxygen sensor element according to Claim 1 or Calim 2, wherein the molar ratio of stabilizing metal oxide(s) to zirconia in said powder mixture is from 4.0:96.0 to 5.5:94.5.

4. An oxygen sensor element according to any one of the preceding claims, wherein the maximum value of the crystal grain size in said solid electrolyte layer is not larger than 1 5 jam but is not smaller than about 3 jam.

5. An oxygen sensor element according to any one of the preceding claims, wherein said reference electrode layer is formed on and in direct contact with a major surface of said substrate, said solid electrolyte layer being formed on said reference electrode layer so as to substantially entirely cover said reference electrode layer and make direct contact with said major surface of said substrate in a marginal region of said electrolyte layer.

6. An oxygen sensor element according to any one of claims 1-4, wherein said solid electrolyte layer is formed on and in direct contact with a major surface of said substrate, said reference electrode layer being formed on said outer surface of said solid electrolyte layer, the oxygen sensor element further comprising a shield coating layer substantially entirely covering said reference electrode layer.

7. An oxygen sensor substantially as herein described with reference to Figs. 1 and 2, Figs. 3 or Fig. 4 of the accompanying drawings.