JPH0510918A - Oxygen-concentration detecting element and manufacture thereof - Google Patents

Abstract

PURPOSE:To improve the durability and the mechanical strength of a detecting element by coating the surface of a measuring electrode with an insulating layer made of a porous, partially stabilized sintered body which has the specific surface area within a specified range and comprises the sintered material of zirconia and metal oxide. CONSTITUTION:In a detecting part 3 of a detecting element 1, a measuring electrode 15, which is exposed to gas to be measured, is provided on one surface of a solid electrolyte 14, and a reference electrode 16, which is exposed to reference gas, is provided on the other surface. The surface of the measuring electrode 15 which is exposed to the gas to be measured is covered with an insulating layer 12. The insulating layer 12 is a porous, partially stabilized sintered body comprising a sintered material whose main raw materials are zirconia and bivalent or trivalent metal oxide such as, e.g. yttria. The specific surface area of the sintered body is more than 3m<2>/g and less than 7m<2>/g. Thus, the transformation temperature between monoclinic system and a tetragonal system can be increased. The occurrences of micro-cracks caused by the transformation are decreased. The same thermal expansion coefficient as that of solid electrolyte can be obtained at the same time. Therefore, cold and hot durability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen concentration detecting element, and more particularly to an insulating layer formed on a measuring electrode of the oxygen concentration detecting element.

[0002]

2. Description of the Related Art An oxygen concentration detecting element comprises a solid electrolyte, a pair of electrodes sandwiching the solid electrolyte, and an insulating layer covering one of the electrodes in contact with exhaust gas.

Conventionally, in order to obtain this oxygen concentration detecting element, a manufacturing method is easy and a high adhesive force can be obtained. Therefore, after the solid electrolyte, the electrode and the insulating layer are laminated, they are simultaneously fired. I've been told.

However, in the conventional method, an insulating layer made of the same material as the solid electrolyte made of partially stabilized zirconia containing monoclinic and tetragonal crystals is provided in order to obtain high adhesion. Method, since the insulating layer is a coarse porous with a lower density than the solid electrolyte, microcracking occurs in the insulating layer due to the volume change during the transformation of monoclinic and tetragonal peculiar to zirconia. Therefore, there is a problem that the mechanical strength such as peeling occurs in several tens to several hundred cycles in the cold heat durability test at the maximum temperature of 1000 ° C., which causes a remarkable decrease in mechanical strength.

Therefore, as disclosed in Japanese Patent Laid-Open No. 60-259952, it has been proposed to use a fully stabilized zirconia composed of only cubic crystals as the raw material of the insulating layer, not the same as the solid electrolyte. It was

[0006]

However, when the raw material for the insulating layer of JP-A-60-259952 is adopted, a partially stabilized zirconia having a thermal expansion coefficient of a solid electrolyte containing a monoclinic crystal is generally used. 7 to 9 × 10 ⁻⁶ / ° C., whereas the coefficient of thermal expansion of fully stabilized zirconia, which is the material of the insulating layer, is 10 × 10 ⁻⁶ / ° C. or more, so peeling due to the difference in thermal expansion coefficient It has been found that this causes a problem in durability.

The present invention has been made in view of the above problems, and provides an oxygen concentration detecting element which is excellent in durability and mechanical strength, and has excellent durability and an easy manufacturing method. The present invention provides a method for manufacturing the same.

[0008]

[Means for Solving the Problems] Therefore, as a result of earnest research, we have noticed that the specific surface area of zirconia, which is a raw material of the insulating layer, is changed. It has been found that it is possible to prevent the volume change due to, and obtain a thermal expansion coefficient almost the same as that of the solid electrolyte.

This is because the insulating layer is composed of partially stabilized zirconia composed of monoclinic and cubic crystals, but the transformation temperature from monoclinic to tetragonal is raised by changing the specific surface area of zirconia. , The operating temperature of room temperature to 100
Since the transformation amount at 0 ° C. can be reduced, the coefficient of thermal expansion can be set to a desired value, and cracks due to volume change can be prevented.

From the above, the present invention is, as the first invention,
A solid electrolyte consisting of partially stabilized zirconia, one surface of which is exposed to the measurement gas and the other surface of which is exposed to the reference gas, and a measurement that is formed on one surface of the solid electrolyte and exposed to the measurement gas The electrode and the reference electrode that is formed on the other surface of the solid electrolyte and is exposed to the reference gas, and that forms a pair with the measurement electrode through the solid electrolyte have a specific surface area of 3 m ² / g.
A porous partially stabilized sintered body composed of a zirconia of less than 7 m ² / g and a sintered material containing a divalent or trivalent metal oxide such as yttria as a main raw material, which is a measuring electrode. And an insulating layer covering the surface exposed to the measurement gas.

As a second aspect of the present invention, a measurement electrode is exposed on one surface of a solid electrolyte of partially stabilized zirconia before sintering so as to be exposed to a measurement gas, and another surface of the solid electrolyte is measured. A first step of forming each reference electrode so as to be exposed to a gas, and a specific surface area of 3 m ² / g or more,
A zirconia of less than 7 m ² / g, and a sintered material mainly composed of a divalent or trivalent metal oxide such as yttria, which is formed on the measurement electrode,
The second step of forming a porous insulating layer so that the measuring electrode is not directly exposed to the measuring gas, and the solid electrolyte is co-fired with the measuring electrode, the reference electrode and the insulating layer to integrally fire the oxygen concentration detecting element. And a third step of obtaining the oxygen concentration detecting element.

[0012]

By adopting the first invention, the specific surface area of zirconia, which is the main raw material of the insulating layer, is 3 m ² / g or more,
Since the amount was less than 7 m ² / g, it was possible to mix the divalent or trivalent metal oxide between the zirconia crystals in an appropriate amount, so that the insulating layer was partially stabilized with monoclinic and cubic crystals. Although zirconia was used, the transformation temperature between the monoclinic crystal and the tetragonal crystal could be raised by changing the physical properties of zirconia by mixing the metal oxide between the zirconia crystals. Therefore, the generation of microcracks caused by this transformation can be significantly reduced, and the thermal expansion coefficient at 1000 ° C. can be set to 7 to 9 × 10 ⁻⁶ / ° C., and a thermal expansion coefficient almost the same as that of the solid electrolyte can be obtained. Therefore, it was possible to improve the cold heat durability.

By adopting the second invention, even if the solid electrolyte, the measuring electrode, the reference electrode and the insulating layer are simultaneously fired, the main raw material of the insulating layer has a specific surface area of zirconia of 3 m ² / g or more, Since it is less than 7 m ² / g, the difference in thermal expansion coefficient between the solid electrolyte and the insulating layer can be almost eliminated, and an oxygen concentration detection element having excellent durability can be provided easily.

[0014]

By adopting the present invention, an oxygen concentration detecting element having excellent durability and mechanical strength can be obtained, and at the same time, a method of manufacturing an oxygen concentration detecting element having excellent durability and easy manufacturing method. Could be provided.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a partial sectional view of an oxygen concentration detector applied to this embodiment. In FIG. 1, 1 is an oxygen concentration detecting element, 2 is a housing having a protective cover 4 for preventing the detecting portion 3 of the oxygen concentration detecting element 1 from being directly exposed to exhaust gas. The oxygen concentration detecting element 1 is assembled and fixed to the housing 4 by a method such as thermal caulking through a powder 5 such as talc.

A lead wire 7 whose one end is connected to the terminal electrode portion 6 of the oxygen concentration detecting element 1 by brazing or the like is taken out to the outside of the oxygen concentration detector main body. Figure 2 shows
It is a development view showing one example of the oxygen concentration detection element 1 of the present example.

In FIG. 2, reference numeral 10 denotes a rectangular cylindrical support body made of alumina porcelain and having through holes with both ends open. The opening end surface 10a of the through hole on the side of the support body 10 that is exposed and exposed to the exhaust gas is an inclined surface that is continuous from one of the side surfaces, and when viewed from the side, this tip portion has a wedge shape. It has become. At the other end of the through hole, an air introduction hole 10b for introducing the atmosphere is provided.

Reference numeral 11 is a heating element mainly made of a white metal material, and a support 1 is provided with an insulating layer 12 made of alumina or the like interposed therebetween.
The heat generating portion 11a of the heat generating element 11 is laminated so as to completely cover the surface facing the inclined surface of 0, that is, the opening 10a as seen in a projection view. Further, in the heating element 11, the insulating layer 13 made of the same material as the insulating layer 12 is laminated on the portion excluding the terminal electrode portions 11b and 11c.

Reference numeral 14 is, for example, a solid electrolyte made of partially stabilized zirconia obtained by sintering zirconia to which yttria, which is one kind of metal oxide, is added. And is exposed to the measurement gas, and the other surface is exposed to the reference gas via the reference electrode 16. Further, the solid electrolyte 14 is laminated so as to seal the open end portion 10 a of the through hole of the support body 10 via the reference electrode 16.

The reference electrode 16 is connected to the reference electrode terminal 17 through a through hole 14a formed at the end of the solid electrolyte 14, and the measuring electrode 15 is connected to the measuring electrode terminal 18
Respectively connected to.

Reference numeral 19 is a protective layer which is an insulating layer which prevents the measurement electrode 15 from being directly exposed to the measurement gas. The protective layer 19 has a specific surface area of 4.1 m ² / g and a particle size of 0.93 μm.
m, the amount of yttria added, which is one of the metal oxides, is 5mo
It is formed of partially stabilized zirconia obtained by firing a ceramic material of 1%.

Next, a method of manufacturing the oxygen concentration detecting element of the present invention will be described below. The shape of the support 10 is determined by injection molding or the like. The insulating layer 1 is formed on the side surface of the support 10.
2, 13 and the heating element 11 are formed by a printing method.

The solid electrolyte 14 is laminated on the support 10 after the electrodes 15 and 16 and the protective layer 19 are formed on both sides of the solid electrolyte 14 by a printing method. After that, the oxygen concentration detection element of this embodiment can be obtained by simultaneous firing.

In this embodiment, the amount of yttria added is 5 mol%, but the amount of yttria added is 5 to 7%.
mol% is preferred. This is 5 mol% yttria
If it is less, the amount of yttria mixed between the crystals of zirconia is too small, and like the conventional zirconia raw material, there is almost no change in the transformation temperature between the monoclinic crystal and the tetragonal crystal, and the solid electrolyte and The difference in thermal expansion coefficient between the two becomes large. Further, when yttria is larger than 7 mol%, the intermixing between zirconia crystals becomes too much,
The transformation temperature rises too much, and the difference in coefficient of thermal expansion with the solid electrolyte also becomes large. Therefore, the best amount of yttria added is 5 to 7 mol%.

When the obtained oxygen concentration detecting element 1 was subjected to a thermal durability test, good results could be obtained without peeling of the protective layer. By changing the composition of the protective layer used in the oxygen concentration detecting element of the present invention, the cold heat durability of the obtained oxygen concentration detecting element was evaluated.

The results are shown in Tables 1 and 2. Here, the specific surface area of zirconia, which is the main raw material of the protective layer,
The measurement was carried out by the BET method using a Flowsorb 2300 type (manufactured by Shimadzu).

The particle size is a value obtained by measurement with Microtrac (manufactured by Nikkiso Co., Ltd.). The porosity of the calcined partially stabilized zirconia was measured by mercury porosimetry.

The crystalline composition from the integrated intensity of X-ray diffraction peak, M / C = _{[I M (111) + I M} (11 1) ] / [I _{M (111)} +
I _{M (111)} + I _{C (111)} ] (M: monoclinic, C: cubic, I:
It was calculated by the integrated intensity).

The cold heat durability test was carried out under the condition of one cycle of 20 minutes as an environmental change of 1000 ° C. × 12 minutes (in combustion gas) and 200 ° C. × 8 minutes (left in the atmosphere). And
A sample without peeling even after 1000 cycles was judged to be good.

[0030]

[Table 1]

[0031]

[Table 2]

As is clear from Tables 1 and 2,
When the specific surface area of zirconia is 7 m ² / g or more (Sample N
o. 14), 15) and 16), the cracks developed and the protective film peeled off.

On the other hand, the specific surface area of zirconia is 6
m ² / g or more and less than 7 m ² / g (Sample No. 1
2, 13), the protective layer was not cracked or peeled up to 1000 cycles, and in particular, when the specific surface area was 6 m ² / g or less (Sample Nos. 1 to 11), protection was performed even after 3000 cycles. It has been found that an oxygen concentration detector having an extremely good protective film can be obtained without causing layer breakage or the like.

The specific surface area of zirconia is 3.0 m ²
If it is less than / g, the sinterability is significantly reduced, and it is very difficult to form a protective layer. FIG. 3 shows the thermal expansion cooling curve with respect to the temperature of the protective layer obtained by changing the specific surface area of each zirconia.

Here, A has a specific surface area of 7 m ² / g, B has a specific surface area of 5.5 m ² / g, and C has a specific surface area of 4.1 m ^2.
/ G, D shows the case where the specific surface area is 3.0 m ² / g. As is clear from FIG. 3, as the specific surface area of zirconia decreases, the transformation temperature between the monoclinic crystal and the tetragonal crystal shifts to the high temperature side, so that the transformation amount of 500 to 1000 ° C. can be reduced and Hysteresis due to volume changes during temperature and cooling could also be reduced.

In the above embodiment, the protective layer obtained according to the present invention was applied to the oxygen concentration detecting element in which the detecting portion 3 has a wedge shape. However, the present invention is not limited to the wedge shape, and for example, the detecting portion may be used. However, it is not particularly limited to a cylindrical shape, a quadrangular prism shape, or the like.

Further, in the above-mentioned embodiment, the present invention is applied to the laminated type oxygen concentration detecting element, but it is also applicable to the cup type oxygen concentration detecting element having no support. In addition, although the insulating layer of the present invention is used as the protective layer of the oxygen concentration battery type oxygen concentration detecting element in the above-mentioned embodiments, it can be applied to the diffusion layer of the limiting current type oxygen concentration detecting element.

Although yttria is used as the divalent or trivalent metal oxide in the above embodiment, the metal oxide is not limited to yttria, and may be magnesia, calcia or the like.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an oxygen concentration detector used in an embodiment of the present invention.

FIG. 2 is a development view of an oxygen concentration detection element of the present invention.

FIG. 3 is a characteristic diagram showing a thermal expansion cooling curve with respect to the temperature of the protective layer at each specific surface area.

[Explanation of symbols]

14 Solid electrolyte 15 Measuring electrode 16 Reference electrode 19 Protective layer

Claims (5)

[Claims] 1. A solid electrolyte composed of partially stabilized zirconia, one surface of which is exposed to a measurement gas, and a measurement electrode which is formed on one surface of the solid electrolyte and exposed to the measurement gas, A reference electrode formed on the other surface of the solid electrolyte and forming a pair with the measurement electrode through the solid electrolyte, zirconia having a specific surface area of 3 m ² / g or more and less than 7 m ² / g, and divalent or trivalent A porous partially stabilized sintered body made of a sintered material mainly composed of the metal oxide of
An oxygen concentration detecting element comprising an insulating layer covering a surface of the measuring electrode exposed to the measuring gas. 2. The coefficient of thermal expansion of the solid electrolyte is 7 to 9 ×.
The oxygen concentration detecting element according to claim 1, wherein the oxygen concentration detecting element is 10 ^-6 / ° C. 3. The divalent or trivalent metal oxide is 5
The oxygen concentration detection element according to claim 1, wherein the oxygen concentration detection element is -7 mol% yttria. 4. The specific surface area of the zirconia is 3 m ² / g.
The oxygen concentration detecting element is 6 m ² / g or less. 5. A measuring electrode is exposed on one surface of a solid electrolyte made of partially stabilized zirconia before sintering so as to be exposed to a measuring gas, and is opposed to the measuring electrode on the other surface of the solid electrolyte. As described above, the first step of forming the reference electrode, the zirconia having a specific surface area of 3 m ² / g or more and 7 m ² / g or less, and the sintering material containing a divalent or trivalent metal oxide as a main material are By forming on the measurement electrode so that the measurement electrode is not directly exposed to the measurement gas,
It comprises a second step of forming a porous insulating layer, and a third step of co-firing the measuring electrode, the reference electrode and the insulating layer with the solid electrolyte to integrally fire them to obtain an oxygen concentration detecting element. A method for manufacturing an oxygen concentration detecting element, comprising: