JP2000292405A - Oxygen sensor element and method of manufacturing the same - Google Patents

Abstract

(57) [Summary] Regarding an electrode layer for an oxygen sensor element, an electrode layer in which ceramics are simply dispersed cannot completely suppress sintering of metal particles, and a more severe element response recently required is required. There was a problem that it was not yet possible to cope with the properties and durability. In an oxygen sensor element in which a pair of metal electrode layers is formed on both surfaces of a zirconia solid electrolyte, the size of metal particles constituting the electrode layer is 0.5 to 7.0 μm.
Wherein these metal particles are Zr and / or Ce
An oxygen sensor element partially buried in the surface of the solid electrolyte with a ceramic layer of an oxide mainly composed of

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an improvement in an oxygen sensor element, and more particularly to an oxygen sensor element having an electrode layer with improved gas responsiveness and durability.

[0002]

2. Description of the Related Art Conventionally, in order to control an air-fuel ratio from an internal combustion engine or the like, an oxygen sensor element has been used to detect the oxygen concentration in a measurement gas such as exhaust gas.

In such an oxygen sensor element, an electrode layer of, for example, platinum or the like is formed on the inner and outer surfaces of a cylindrical solid electrolyte made of zirconia, and air is exhausted to an inner reference electrode and exhausted to an outer gas electrode to be measured. In general, a gas is supplied, the output voltage obtained at that time is measured, and the oxygen concentration in the exhaust gas is detected.

The electrodes of such an oxygen sensor element have been formed by a method such as physical vapor deposition such as a chemical plating method or a sputtering method. However, the electrodes formed by these methods have the disadvantage that gas permeability is poor due to their dense nature, resulting in poor gas responsiveness. Although the gas permeability of the electrode layer is improved by heat treatment, the sintering of the metal particles of the electrode reduces the three-phase interface of the gas phase / solid electrolyte / metal particles, and conversely deteriorates the gas responsiveness. Or the adhesion of the electrode to the electrolyte is reduced.

[0005] In recent years, there has been a demand for an oxygen sensor having an excellent gas responsiveness and an excellent durability as compared with the conventional oxygen sensor. With respect to such an oxygen sensor element, JP-A-1-18445 discloses a method for improving the above problem.
No. 7, the Al ₂ O ₃ , BaO, C
An electrode structure in which heat-resistant ceramic particles such as eO ₂ and MgO are dispersed has been proposed.

On the other hand, in Japanese Patent Application Laid-Open No. 2-276958, a paste mainly containing an organometallic compound such as a metal alkoxide containing platinum powder containing Li, Na, K and Be is applied and thermally decomposed. A technique has been proposed in which ceramics are dispersed to form an electrode having uniform fine holes.

[0007]

Problems to be Solved by the Invention
Nos. 7 and 2-276958,
In an electrode structure in which ceramic particles are dispersed in metal particles, the gas permeability is improved due to the presence of many fine pores in the electrode layer. As a result, gas responsiveness and durability are improved compared to conventional devices. Excellent.

However, in the electrode layer in which the ceramic is simply dispersed as in the above method, the sintering of the metal particles cannot be completely suppressed. There was a problem that it could not cope sufficiently.

[0009]

Means for Solving the Problems As a result of studying the above problems, the present inventor has found that metal particles having a predetermined size to form an electrode are partially formed on the surface of an electrolyte using a ceramic having oxygen ion conductivity. It has been found that the above-mentioned problems relating to gas responsiveness and durability can be solved by burying them in a specific manner, leading to the present invention.

That is, the present invention provides an oxygen sensor element in which a pair of metal electrode layers are formed on both surfaces of a zirconia solid electrolyte, wherein the metal particles constituting the electrode layer have an average particle diameter of 0.5 to 7.0 μm. The metal particles are partially embedded in a ceramic layer of an oxide containing Zr and / or Ce as a main component provided on the surface of the solid electrolyte.

The present invention also relates to a method for manufacturing an oxygen sensor element in which a pair of metal electrode layers are formed on the inner and outer surfaces of a zirconia solid electrolyte, wherein the metal particles and Zr and / or Ce are contained. After applying a mixed solution obtained by mixing a solution of an organometallic compound to the surface of the electrolyte, it is thermally decomposed to form an oxide ceramic layer, and the metal particles are partially embedded in the ceramic layer. .

[0012]

According to the present invention, since the metal particles are embedded in the oxide ceramic layer provided on the surface of the electrolyte, the adhesion of the metal particles to the electrolyte is greatly enhanced and, at the same time, the three phases accompanying the embedding of the metal particles are achieved. Since the interface is increased and the metal particles come into contact with the ceramic having excellent oxygen ion conductivity, the rate of the gasification reaction of oxygen ions from the electrolyte or the ionization reaction of oxygen gas is promoted. Therefore, in the present invention, it is possible to obtain a fixing force of metal particles to the electrolyte and excellent gas responsiveness and durability, which cannot be obtained by the conventional device.

In the present invention, in order to embed the metal particles on the surface of the electrolyte, the metal particles such as platinum group metals are used, for example, Z
A method of dispersing in a solution of an organometallic compound containing r, Y or the like, applying the solution to the surface of the electrolyte, and thermally decomposing in an atmosphere containing oxygen gas in a temperature range of 1000 to 1500 ° C. is used. As a result, an electrode structure in which metal particles are partially or mostly buried in the (Zr, Y) O ₂ layer generated from the organometallic compound on the electrolyte surface is easily formed. At this time, in order to successfully embed the metal particles in the ceramic layer, the size of the metal particles after the heat treatment is 0.5 to
It is desirable to be 7.0 μm. In addition, according to this method, a part of the solution of the organometallic compound attached to the surface of the metal particle causes the (Zr, Y) O ₂
Since they are deposited on the surface of the particles as fine particles of, they have both an effect of suppressing sintering of the metal particles under element use conditions and an effect of increasing gas permeability.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, an electrode layer 5 of an oxygen sensor element according to the present invention is formed by forming a ceramic layer 4 having ionic conductivity or electronic conductivity and ionic conductivity on the surface of a solid electrolyte 3. The ceramic layer 4 has an electrode structure in which the metal particles 2 are partially or mostly embedded.
For this reason, the metal particles 2 are firmly fixed to the electrolyte 3, and the separation of the metal particles 2 due to sintering is suppressed, and at the same time, the three-phase interface of the gas phase / electrolyte / metal particles increases, and the gas responsiveness is increased. .

Further, in the electrode of the present invention, as shown in FIG. 1, a large number of fine ceramic particles 1 exist on the surface of metal particles 2. In order to obtain the electrode layer 5 including the ceramic layer 4 in which the metal particles 2 are embedded, the average particle diameter of the metal particles 2 is preferably 0.5 to 7.0 μm. When the average particle diameter of the metal particles 2 is smaller than 0.5 μm, the metal particles 2 are almost completely buried in the ceramic layer, and the function as an electrode deteriorates. On the other hand, metal particles 2
When the average particle diameter of the metal particles exceeds 7.0 μm, the degree of embedding of the metal in the ceramic layer 4 is reduced, and the metal particles 2 are easily separated by sintering. For these reasons, the size of the metal particles 2 is 0.5 to 7.0 μm, particularly 1 to 3 μm.
The range of m is desirable.

At this time, as the metal particles 2 of the electrode, rhodium, ruthenium, palladium, gold or an alloy thereof is used in addition to platinum. In order to obtain the metal particles 2 having a predetermined size after the heat treatment, it is preferable that the metal powder used has an average particle diameter in a range of 0.1 to 5 μm. It is necessary to properly select the size after the heat treatment depending on the metal content and the like.

As the ceramic layer 4 for embedding the metal particles 2, a material which is stable even in a reducing atmosphere such as hydrogen or CO, has oxygen ion conductivity, and also has electron conductivity is desirable. From the viewpoint of reducing atmosphere stability and electric conductivity, Y ₂ O ₃ , Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂
ZrO ₂ and / or CeO ₂ at least one containing 5 to 20 mol% in oxide equivalent of O ₃ is desirable. Among these materials, particularly strong adhesion to zirconia electrolyte, and as the ceramic having both electron conductivity and ion _{conductivity, Y 2 O 3, Sm 2} O 3 8~ in total the
A solid solution of ZrO ₂ and CeO ₂ containing 15 mol% is preferred.

On the other hand, as the solid electrolyte 3 of the device of the present invention, ZrO ₂ containing 3 to 20 mol% of oxides of Y ₂ O ₃ , Yb ₂ O ₃ , Sc ₂ O ₃ and Sm ₂ O ₃ is used. It is preferably used. From the viewpoint of the strength of the element, Y ₂ O ₃ ,
Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , CaO in 3 to 8
Molar% ZrO ₂ is desirable. The porous electrode protective layer and the gas diffusion controlling layer provided on the surface of the gas electrode to be measured are made of a material such as zirconia, spinel, or aluminum, and are formed by a plasma spraying method or a sputtering method. Next, as an example of the method for forming an electrode of the present invention, a case where platinum is used as the metal particles 2 and (Zr, Y) O ₂ is used as a ceramic for embedding the metal particles 2 will be described in detail. First, a platinum powder of a predetermined size is added to a solution of octylate containing Zr and Y dissolved in toluene, and the electrolyte sintered body is formed by a known method such as a slurry dipping method, a screen printing method, and a pad printing method. Apply to the surface and slowly dry at around 100 ° C. so that the solution concentrates on the electrolyte surface. Thereafter, 1000 to 1500 in the atmosphere or in an inert gas such as argon gas or nitrogen gas.
Heat treatment is performed for 1 to 3 hours in a temperature range of ° C., and thermal decomposition is performed to form an electrode layer 5 in which some of the platinum particles are embedded in (Zr, Y) O ₂ . When the heat treatment temperature is lower than 1000 ° C., the ceramic generated by the decomposition of the organometallic compound is powdered, and the ceramic layer 4 is not formed. On the other hand, when the processing temperature exceeds 1500 ° C., the sintering of the metal particles 2 proceeds, and the metal particles 2 become larger than 7 μm. As a result, the effect of embedding is weakened, and the gas responsiveness and durability are deteriorated.
The heat treatment temperature is particularly preferably from 1200 to 1400 ° C. from the viewpoint of gas responsiveness and durability.

As the organometallic compound, a fatty acid salt such as a naphthenate or an acetylacetona complex can be used in addition to the octylate. Also, as an organic solvent,
In addition to toluene, acetylacetone is used.

The amount ratio of the organometallic compound is such that the amount of ceramic formed by thermally decomposing the organometallic compound is 1 to 20% by weight, preferably 5 to 1% by weight based on the amount of metal.
It may be adjusted to be 0% by weight. If the amount of the generated ceramic is less than 1% by weight with respect to the amount of the metal, the metal particles 2 are not sufficiently buried. If the amount exceeds 20% by weight, the amount of the ceramic particles deposited on the surface of the metal particles 2 is reduced. And the gas responsiveness deteriorates.

Next, the oxygen sensor element of the present invention will be described. In the device of the present invention, as shown in FIG. 2, a reference electrode 6 is formed on an inner surface of a cylindrical solid electrolyte 3 whose one end is sealed, and an electrode layer including a measurement gas electrode 7 is formed on an outer surface. Although not described above, at least one or more porous electrode protective layers made of spinel, zirconia, or alumina, or a gas diffusion-controlling layer are formed on the surface of the outer electrode layer.

It is desirable that both the measurement gas electrode 7 and the reference electrode 6 have the structure of the electrode layer 5 of the present invention shown in FIG. 1, but from the viewpoint of gas responsiveness, the performance is dominant. There is a sufficient effect even if it is applied only to the measurement gas electrode 7.

In order to fabricate such an element, a cylindrical molded body made of an electrolyte is molded by a known method such as extrusion molding, press molding, isostatic pressing, and the like, and is heated to 1400 to 1600 ° C. in the atmosphere. To produce a cylindrical sintered body made of an electrolyte, and then the electrode layer 5 is formed according to the method of the present invention.
To form At this time, the porous electrode protection layer 8 or the gas diffusion rate-controlling layer can be formed by a plasma spraying method or a sputtering method. Alternatively, a solution of a metal organic salt containing platinum powder or the like is applied to the inner and outer surfaces of the cylindrical molded body in advance to a predetermined thickness and a predetermined area, and then fired simultaneously with the cylindrical molded body to form the electrode layer 5. It is also possible to produce a cylindrical sintered body. Also, a porous electrode protection layer 8,
Alternatively, the gas diffusion-controlling layer may be formed in advance at the stage of the molded body, and may be manufactured by firing simultaneously with the cylindrical molded body. On this occasion,
As the firing temperature, a temperature of 1300 to 1500 ° C. or less is preferable.

The electrode layer 8 of the present invention can also be applied to a flat oxygen sensor element in which a heater is embedded as shown in FIG. In the flat element of the present invention, the reference electrode 6 is provided on one surface of the solid electrolyte 3 and the measurement gas electrode 7 is provided on the other surface.
Are formed, and an electrode protection layer 8 is formed on the surface of the measurement gas electrode 7. Further, a platinum heater layer 9 for heating the element is embedded in the element via an insulator layer 10 made of alumina, spinel, forsterite or the like. In order to produce an element having such a shape, first, a green sheet made of zirconia is produced by, for example, a doctor blade method or an extrusion molding method. Thereafter, the electrode layer 5 is formed on both surfaces of the green sheet according to the method of the present invention. On the other hand, a heater layer 9 of platinum is formed on the surface of the green sheet via an insulator layer 10 such as alumina or spinel by screen printing or the like. Thereafter, an integrated laminated body in which the above-described sheets are laminated so as to have the shape shown in FIG. 3 is produced, and the resultant is fired in the air at 1300 to 1500 ° C. to produce a flat element having a heater embedded therein. At this time, the outer electrode layer 5
The electrode protection layer 8 and the gas diffusion controlling layer provided on the surface of
It can be manufactured by a plasma spraying method or a sputtering method. Alternatively, after forming the electrode layer 5 on both surfaces of the green sheet according to the method of the present invention, and further applying a powder of alumina, spinel, zirconia or the like by screen printing or the like,
The electrode protection layer 8 and the gas diffusion rate-controlling layer may be formed by firing simultaneously with the laminate.

The oxygen sensor element described above is an example of the present invention. The element of the present invention only needs to have the electrode layer 5 of the present invention formed on the outside and inside of the solid electrolyte 3 and has a cylindrical or cylindrical shape. It does not mean a shape such as a flat plate shape.
Further, the present invention may be a fitting type cylindrical element into which a heater is inserted, or a cylindrical and flat oxygen sensor element into which a heater is integrated.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in accordance with embodiments.

Example 1 ZrO ₂ powder containing 5 mol% Y ₂ O ₃ , several kinds of platinum powder having an average particle diameter in the range of about 0.05 to 10 μm, and organic powder containing Y, Zr and Ce respectively A solution in which a metal salt was dissolved was prepared.

First, PVA was added to ZrO ₂ powder to form germ soil, and a hollow cylindrical molded body having one end sealed by extrusion was produced. The molded body is heated at 1600 ° C. in the atmosphere.
For 5 hours to produce a cylindrical dense solid electrolyte 3.

On the other hand, after the heat treatment, 10 mol% of Y ₂ O ₃ mol /
90 mol% ZrO ₂ and 10 mol% Y ₂ O ₃ mol / 5
Additional platinum powder was added and sufficiently stirred to prepare a slurry, respectively in a solution containing the above mentioned organic metal was adjusted to ceramic mol% CeO _2/85 mol% ZrO ₂ composition. Thereafter, a slurry containing platinum powder is applied to the inner and outer surfaces of the cylindrical electrolyte so as to have a thickness of about 10 to 15 μm, and dried at 100 ° C. for 1 hour. Heat treatment was performed at a temperature of 1 ° C. for 1 hour to decompose the organic metal salt and bury the platinum powder on the surface of the solid electrolyte 3 by the decomposition.

In order to compare the performance of the device, a sample containing a precious metal powder having a catalytic action and a paste containing an organometallic compound was applied to both surfaces of a solid electrolyte 3 formed of cylindrical zirconia having one end closed, and then fired. No. 1 and the solid electrolyte 3 formed body was calcined at 1350 ° C. for 10 hours, and then 6 mol% Y ₂ O ₃ added ZrO
0.7 μm baked paste containing 6% by weight of ₂
Is formed on the outer electrode layer 5.
Sample No. _{2 having} a porous electrode protection layer 8 made of spinel of ₂ O ₃ -MgO was formed. 2 sensor elements were produced.

The gas responsiveness was evaluated by assembling the sensor element thus manufactured in a metal housing, and setting the exhaust gas temperature to 3 using a computer for engine control outside a four-cylinder gasoline engine with a displacement of 1500 cc.
While maintaining the temperature at 50 ° C., the air-fuel ratio in the exhaust gas was switched from 14.0 to 15.4 and vice versa, and the sensor output after switching was switched from 0.6V to 0.3V. Conversely, response times TRL and TL changing from 0.3V to 0.6V
R was measured respectively.

On the other hand, the low-temperature operability of the element includes the temperature of the exhaust gas of an engine having a displacement of 1500 cc, and
The temperature was set to 0 ° C., and thereafter, the exhaust gas temperature was reduced to a temperature at which the output voltage of the element became weak and feedback control became impossible, and the exhaust gas temperature at that time was measured. It can be said that the lower the temperature, the better the low-temperature operability of the element.

The results are shown in Table 1. At this time, in the present invention, regarding the diameter (size) of the metal particles 2, the maximum diameter of the metal particles 100 on the electrode surface was measured with a scanning electron microscope, and the average value was defined as the metal particle diameter. .

[0034]

[Table 1]

According to Table 1, the size of the embedded metal particles 2 is smaller than 0.5 μm. 3 is metal particle 2
Is buried in the ceramic layer 4 and the gas responsiveness and the low-temperature operability are lower than those of the conventional devices (samples No. 1 and No. 2). The metal particle size is
Sample No. exceeding 7.0 μm. In No. 13, on the contrary, the degree of embedding of the particles in the ceramic layer decreases, and similarly, the gas responsiveness and the low-temperature operability also deteriorate. In addition, the sample No. whose heat treatment temperature is lower than 1000 ° C. Sample No. 5 and Sample No. 5 in which the heat treatment temperature exceeded 1500 ° C. 13 also shows poor performance. On the other hand, in the present invention, the heat treatment temperature is 1
000 to 1500 ° C., the metal particle diameter is 0.5 to 7.0 μ
It can be seen that all the samples in the range of m have superior gas responsiveness and low-temperature operability to the comparative product.

In the embodiment of the present invention, the sample No. 15 and No. 16 shows that both the responsiveness and the low-temperature operability are superior to the sample without Ce addition. This seems to be due to the high electron conductivity of the Ce-added ceramic.

Example 2 Example of the present invention used in Example 1 (sample No. 6, N
o. 7 and No. 7 8) and Comparative Examples (Samples No. 1 and No. 2) were evaluated for gas responsiveness and low-temperature operability after 2,000-hour life cycle durability according to Example 1. Table 2 shows the results.

[0038]

[Table 2]

As shown in Table 2, with respect to gas responsiveness, the TRL and the TRL of the comparative examples (samples No. 17 and No. 18) were used.
Each TLR showed a value larger than 80 ms. In addition, the low-temperature operability was higher than 300 ° C. in all cases. In contrast, the embodiment of the present invention (Sample No. 1)
9, No. 20 and no. 21) TRL, TLR
Showed a value smaller than 30 ms. As for the low-temperature operability, all of the products of the present invention were lower than 250 ° C.

From the above results, it can be sufficiently understood that the oxygen sensor element of the present invention is superior in durability and low-temperature operability as compared with the conventional oxygen sensor element.

[0041]

According to the present invention, as described above, a solution composed of an organometallic compound in which metal particles forming an electrode of an oxygen sensor element are dispersed is applied to the surface of a solid electrolyte and is thermally decomposed. The particles form an electrode layer embedded in the electrolyte surface. In this electrode layer, since the metal particles are firmly fixed to the electrolyte, the gas responsiveness is improved as compared with the conventional device, and at the same time, the separation of the metal particles from the electrolyte due to sintering of the metal is prevented, Durability also improves dramatically.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an electrode structure in an oxygen sensor element of the present invention.

FIG. 2 is a sectional view of a main part showing an oxygen sensor element of the present invention.

FIG. 3 is a sectional view of a main part showing an oxygen sensor element of the present invention.

[Explanation of symbols]

 1: Precipitated ceramic particles 2: Metal particles 3: Solid electrolyte 4: Ceramic layer 5: Electrode layer 6: Reference electrode 7: Measurement gas electrode 8: Electrode protection layer 9: Heater layer 10: Insulator layer

Claims (2)

[Claims] 1. An oxygen sensor element comprising a zirconia solid electrolyte and a pair of metal electrode layers formed on both surfaces thereof, wherein an oxide ceramic layer containing Zr and / or Ce as a main component is formed on the surface of the solid electrolyte. An oxygen sensor element comprising an electrode layer formed by partially burying metal particles having an average particle diameter of 0.5 to 7.0 μm in the ceramic layer. 2. A method for manufacturing an oxygen sensor element in which a pair of metal electrode layers are formed on the inner and outer surfaces of a zirconia solid electrolyte, wherein a metal particle and a solution of an organometallic compound containing Zr and / or Ce are mixed. A method for producing an oxygen sensor element, comprising forming a ceramic layer of an oxide in which metal particles are partially embedded by applying a mixed solution to an electrolyte surface and then thermally decomposing the mixed solution.