JP2879965B2 - Method for manufacturing oxygen sensor element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oxygen sensor element and a method for manufacturing the same.
In particular, the present invention relates to a technique for improving low-temperature operability and responsiveness of an oxygen sensor element for measuring an amount of oxygen contained in exhaust gas of an internal combustion engine, a boiler, or the like.

(Background Art) Conventionally, using a solid electrolyte made of oxygen-ion conductive zirconia porcelain, for example, by the principle of an oxygen concentration cell or the like, the measured object discharged from a combustion device in an internal combustion engine of an automobile or the like or an industrial furnace. It is known to detect the oxygen concentration of exhaust gas as a gas and control the air-fuel ratio or combustion state of such an internal combustion engine or the like.

In this type of oxygen sensor, which is an oxygen concentration detector of this type, as a sensor element, a solid electrolyte having a bottomed cylindrical shape or a plate-like shape is made of or contains a catalytic metal such as platinum. At least one pair of electrodes is provided, and at least one of the electrodes is brought into contact with a gas to be measured such as exhaust gas.

In addition, as an electrode provided in this type of oxygen sensor element, a catalyst metal powder such as platinum and a solid electrolyte body are used in order to improve durability under severe use conditions such as long term or extremely high temperature. A paste is prepared by mixing with the zirconia powder of the co-fabric, and this paste is screen-printed on a sensor element (element body) made of stabilized or partially stabilized zirconia, etc., and simultaneously integrated with the sensor element. Cermet electrodes formed by firing are known.

However, such a cermet electrode is generally fired simultaneously with the sensor element at a high temperature of at least 1000 ° C., preferably at least 1300 ° C., in the air in order to improve its durability. Therefore, there is a problem that oxidation is induced on the surface of the electrode, and in the electrode obtained by integrally firing, the firing temperature of the solid electrolyte material is relatively high, so that sintering of a catalyst metal such as platinum is likely to proceed. In addition, since the sintering aid is usually added to the solid electrolyte material, such a sintering aid enters the electrode layer during sintering, and covers the catalytic metal constituting the electrode as an impurity. There is a problem. For these reasons, the cermet electrode in the conventional oxygen sensor element has a low catalytic activity and lacks porosity, which causes a problem that the low-temperature operability and responsiveness of the oxygen sensor element are poor. I was holding it.

In particular, in recent years, exhaust gas regulations have become strict, and in automobiles, oxygen sensors that operate even during idling, and oxygen that operates stably even in low-temperature parts behind the exhaust pipe due to restrictions on the sensor mounting position and the like. Although a sensor element has been required, the conventional oxygen sensor element has not been able to operate stably under exhaust gas having a low temperature of, for example, 400 ° C. or less.

(Problem to be Solved) Here, the present invention has been made in view of such circumstances, and a problem to be solved is to operate stably even in a low-temperature gas to be measured and have a response To realize an oxygen sensor element with excellent
Another object of the present invention is to provide a method for advantageously producing an oxygen sensor element having such excellent characteristics.

(Solution) In order to solve the above-mentioned problems, the present invention provides an oxygen sensor element having at least a pair of electrodes supported by an oxygen ion-conductive zirconia solid electrolyte body. At least,
The electrode exposed to a measurement gas, together make a porous cermet electrode which mainly of catalytic metal and ZrO _2, in the porous electrode, the ZrO ₂ component, monoclinic ZrO ₂ or partially stabilized zirconia Characterized in that it is present as zirconia particles having a particle diameter of 2 μm or less in a ratio of 5 to 60 parts by weight per 100 parts by weight of the catalyst metal.

In the oxygen sensor element according to the present invention,
As is well known, monoclinic ZrO ₂ constituting the zirconia particles is formed from pure zirconia without adding a stabilizer, and partially stabilized zirconia is CaO, Y It is formed by adding at least one of ₂ O ₃ and Yb ₂ O ₃ to zirconia at a ratio of 4 mol% or less.

Further, the present invention relates to a method for manufacturing an oxygen sensor element in which at least a pair of electrodes is supported on an oxygen ion-conductive zirconia solid electrolyte in order to advantageously obtain an oxygen sensor element having excellent low-temperature operability and responsiveness. Of the at least one pair of electrodes, at least one of the electrodes exposed to the gas to be measured has a particle diameter of catalyst metal powder and monoclinic ZrO _2.
After applying an electrode forming material prepared from a zirconia powder having a particle diameter of 0.2 to 2.5 μm or a partially stabilized zirconia having a particle diameter of 0.2 to 2.0 μm to the solid electrolyte body, by firing, the porous SUMMARY OF THE INVENTION The gist of the present invention is a method for manufacturing an oxygen sensor element, including a step of forming a quality cermet electrode.

Furthermore, in the present invention, the porous cermet electrode formed by firing the oxygen sensor element obtained as described above is treated with (a) an acid treatment and / or (b) a reducing gas. The gist of the present invention is to provide a post-treatment of performing a heat treatment under at least one gas atmosphere of an atmosphere and an unsaturated hydrocarbon atmosphere. Characteristics such as response and responsiveness can be further enhanced.

Incidentally, even in such a method of manufacturing an oxygen sensor element, the zirconia powder may be monoclinic ZrO ₂ or
4 mol% of at least one of CaO, Y ₂ O ₃ and Yb ₂ O ₃
What consists of partially stabilized zirconia formed by adding at the following ratios is advantageously used.

First, FIG. 1 shows a typical structure of an oxygen sensor element to which the present invention is applied in a sensor structure based on the principle of an oxygen concentration cell.

That is, the oxygen sensor element shown in FIG.
A plate-shaped solid electrolyte body 2 formed integrally by a known laminating technique and having a predetermined width and a narrow width.
A measurement electrode 4 and a reference electrode 6 are respectively provided on both sides of the sample, and a measurement gas such as exhaust gas is brought into contact with the measurement electrode 4 through the porous protective layer 8. On the side of the solid electrolyte body 2 on which the reference electrode 6 is provided, a spacer 12 for forming an air passage 10 and a plate-like lid member 14 are laminated to form an integral structure. The air passage 10 formed inside such an element
Is opened at the base end side of the element and is allowed to communicate with the atmosphere. Air introduced through the air passage 10 is supplied to the reference electrode 6 as a reference gas having a predetermined reference oxygen concentration. It can be contacted.

In addition, the oxygen sensor element having such a structure includes a lid member.
Outside the heater element 14, a heater layer in which a heater element 16 is sandwiched between electric insulating layers 18 and 20 made of alumina or the like is laminated and integrated together with an airtight layer 22 located outside the heater element. By causing the heater element 16 to generate heat by externally supplying power to the element 16, the buried portions of the electrodes 4 and 6 of the oxygen sensor element are heated to a predetermined temperature.

In the oxygen sensor element having such a structure, as is well known, the oxygen concentration in the atmosphere (gas to be measured) brought into contact with the measurement electrode 4 through the porous protective layer 8 and the reference electrode According to the present invention, the oxygen concentration in the target gas to be measured is detected according to the electromotive force generated based on the difference from the oxygen concentration in the atmosphere (reference gas) brought into contact with 6. In
At least the electrode 4 in such an oxygen sensor element that is brought into contact with the gas to be measured is a predetermined porous cermet electrode.

The present invention is not limited to the sensor element having the structure as shown in FIG. 1, but as is well known, a sensor element using a zirconia solid electrolyte having a bottomed cylindrical shape. It is needless to say that the present invention can also be applied to a sensor element having a known structure based on various measurement principles such as a limiting current method and a polarographic method. is there.

Incidentally, as the oxygen ion-conductive solid electrolyte body (2) constituting such an oxygen sensor element, in addition to the plate-like shape as described above using a known ZrO ₂ solid electrolyte material, A known shape such as a shape can be adopted, for example, CaO, Y ₂ O ₃ , Yb ₂ O ₃ or the like, to which one or more predetermined stabilizers are added, oxygen ion conductivity mainly composed of ZrO ₂ From a stabilized or partially stabilized zirconia material according to an appropriate molding technique. In addition, such a solid electrolyte material also has a predetermined sintering aid,
For example, clay such as kaolin, SiO ₂ , Al ₂ O ₃ , Fe ₂ O ₃ or the like is appropriately mixed.

In the oxygen sensor element shown in FIG.
2. The lid member 14 and the airtight layer 22 can be formed by using a known ceramic material.
Although the porous protective layer 8 is formed using the same type of material as the solid electrolyte body 2 in accordance with a known method, The agent may or may not be included at all. Thus, for forming the porous protective layer 8, it is particularly effective to use a ZrO ₂ solid electrolyte material stabilized with a stabilizer such as Yb ₂ O _3. The amount of the stabilizer such as Yb ₂ O ₃ in the layer 8 is made larger than that of the solid electrolyte 2 so that the porous protective layer 8 and the solid electrolyte 2
It is preferable to make the crystal phase of ZrO ₂ different from that of ZrO ₂ . More specifically, the crystalline phase of ZrO ₂ in the porous protective layer 8 is cubic only, or mostly cubic, and stable in thermal cycling or thermal shock, and a small amount of tetragonal or monocrystalline. On the other hand, ZrO ₂ of the solid electrolyte body 2 is made to have a crystal phase containing a clinic crystal.
Most of them are composed of a tetragonal system, a mixed phase of tetragonal system, monoclinic system and cubic system, or a mixed phase of monoclinic system and cubic system. Is desirable.

Further, of the measurement electrode 4 and the reference electrode 6 formed at predetermined positions on the solid electrolyte body 2, an electrode forming material that provides at least the electrode (4) exposed to the gas to be measured is a catalyst that serves as an electrode conductor. It is composed of a metal powder and a zirconia powder. A known catalyst metal is used, for example, platinum or an alloy of platinum with a metal such as nickel, silver, gold, rhodium, palladium, iridium, ruthenium, or the like. Further, as the zirconia powder, advantageously, a monoclinic ZrO ₂ powder or CaO, Y ₂ O ₃ , Yb ₂
A partially stabilized zirconia powder obtained by adding one or more components such as O _{3 at} a ratio of 4 mol% or less will be used. Particularly, ZrO ₂ which causes phase transformation, more specifically, room temperature It is preferable to use ZrO ₂ whose lower crystal phase is a single crystal.

Such monoclinic ZrO _2, ie The reason for characteristic improvement by the use of a pure ZrO ₂ without added stabilizing agents, pure ZrO ₂ is sintered than stabilized zirconia additive was added Low sintering, in addition to being able to suppress sintering of the catalyst metal constituting the electrode, in addition to firing the electrode,
During volume expansion based on the phase transition of ZrO ₂ that occurs during cooling, cracks occur in the zirconia (ZrO ₂ ) and catalyst metal that constitute the electrode, and a porous and large surface area electrode is formed. It is estimated that

Therefore, in such an electrode forming material, if the particle size of the zirconia powder constituting the electrode material is too small, it is difficult to reduce the size by the phase transition as described above. Dispersibility (uniform mixing) is poor, and sintering of the zirconia powder easily proceeds, which causes a problem such as a reduction in the fineness effect due to the phase transition as described above. On the other hand, if the particle diameter of the zirconia powder is too large, the adhesion of the electrode obtained by firing to the solid electrolyte body is deteriorated, and in addition to the problem that the durability of the electrode is reduced. As a result, there arises a problem that the collectivity of the catalyst metal powder in the electrode is increased, and the sintering of the catalyst metal proceeds. Therefore, in the present invention, in the case of using a zirconia powder consisting of single-crystalline ZrO _2, the particle size is required to be 0.2 to 2.5 [mu] m, more preferably in the range of 0.2 and 1.5 .mu.m, particularly preferably 0.6 ~
A range of 1.2 μm will be selected. Further, when using a zirconia powder made of partially stabilized zirconia, the particle size needs to be 0.2 to 2.0 μm, more preferably 0.2 to 1.5 μm, particularly preferably 0.6 to 1.2 μm Will be adopted. In addition,
The particle size of the zirconia powder is measured according to a particle size measuring method using a laser scattering method.

In addition, the composition ratio of the catalyst metal powder and the zirconia powder in such an electrode forming material varies depending on the production conditions such as the firing temperature, but if the amount of the catalyst metal powder is small, the conductivity as the electrode is lost, and if the amount is too large, Since the sintering of the catalyst metal powder progresses, the electrode is densified, and the porosity is adversely affected, the zirconia powder is generally 5 to 60 weight parts with respect to 100 parts by weight of the catalyst metal powder. Parts, more preferably 10 to 20 parts by weight of zirconia powder.

Then, using such an electrode forming material composed of the predetermined catalyst metal powder and the zirconia powder, the electrode forming material is formed at a predetermined portion of the solid electrolyte body at least to form an electrode exposed to the gas to be measured. Is applied to a predetermined portion of the solid electrolyte body, a known electrode formation method is appropriately adopted, for example, an electrode formation layer of a predetermined thickness by a screen printing method, a spray method, or the like, on the solid electrolyte body. After the formation, heating and baking (baking) can form a desired porous cermet electrode. More specifically, in the screen printing method, a mixed paste of the above-described catalyst metal powder and zirconia powder is prepared, printed on a solid electrolyte body by a screen printing method, and heated and baked. In the spray method, a zirconia powder is added to a slurry containing a predetermined catalyst metal powder, and the zirconia powder is spray-coated on a predetermined portion of the solid electrolyte, and then heated to bake a desired porous electrode. Can be formed.

The solid electrolyte body to which the electrode forming material is applied may be a fired one, but in the present invention, it is preferably a green sheet or green body before firing. It is desirable that the operation of heating and baking the electrode forming layer (electrode forming material) formed on the solid electrolyte body be performed simultaneously with the firing operation of the solid electrolyte body.

Further, among the electrodes provided on the oxygen sensor element, to form an electrode other than the electrode exposed to the gas to be measured, for example, the reference electrode 6 in the configuration as shown in FIG. According to the conventionally known, plating method, sputtering method, method by thermal decomposition of salt of electrode conductor such as catalyst metal, paste baking method of baking cermet paste of electrode conductor and ceramics on the surface of solid electrolyte body, and After printing or coating such a cermet paste on the solid electrolyte body, an appropriate method is selected and adopted from various methods such as a simultaneous firing method of the cermet paste which is fired together with the solid electrolyte body. However, in the present invention, since the measuring electrode 4 needs to be configured as a cermet electrode, the above-described electrode forming method is not used. Also, so that the one of the simultaneous firing method of paste printing method or a cermet paste is advantageously employed.

In the oxygen sensor element thus obtained, at least an electrode such as the measurement electrode 4 exposed to the gas to be measured is formed as a porous cermet electrode mainly composed of a catalyst metal and ZrO _2. In such a porous electrode, the ZrO ₂ component is present as zirconia particles of 2 μm or less composed of monoclinic ZrO ₂ or partially stabilized zirconia at a ratio of 5 to 60 parts by weight per 100 parts by weight of the catalyst metal. Therefore, the porous structure of such an electrode can be advantageously miniaturized, so that the electrode has a large surface area, effective catalytic activity can be imparted, and low-temperature operability and responsiveness of the oxygen sensor element can be obtained. Can be significantly improved. In order to sufficiently exhibit such an effect, the particle diameter of the zirconia powder (particles) in the fired electrode is not limited to the single-crystal ZrO ₂ or partially stabilized zirconia, According to the SEM (scanning electron microscope) image of the above, it is necessary to be 2 μm or less. Further, the zirconia powder in such an electrode is a single crystal Zr.
In the case of O ₂ , preferably 1.5 μm or less,
More preferably, the particle size is 0.6 μm or less, and in the case of partially stabilized zirconia, the particle size is preferably 1.7 μm or less, more preferably 1.2 μm or less.

In the present invention, the porous cermet electrode obtained by firing the oxygen sensor element thus obtained is subjected to (a) an acid treatment and / or (b) a reducing gas and / or a non-reducing gas. A heat treatment in a saturated hydrocarbon atmosphere is advantageously employed, and the performance of such a treatment can significantly increase the catalytic activity of the porous cermet electrode, i.e., the electrode exposed to the gas to be measured, and It is possible to further improve the low-temperature operability and responsiveness of the sensor element.

The above-mentioned acid treatment (a) is to remove impurities covering the surface of the catalyst metal constituting such an electrode by an acid soaked in a porous electrode, and to carry out the above-mentioned heat treatment (b). ) Is to remove the metal oxide formed on the surface of the porous electrode and to increase the catalytic activity by performing the acid treatment (a) and the heat treatment (b) independently, Although some degree of effect associated with such processing can be expected, it is particularly recommended in the present invention to perform both of these processings (a) and (b). However, when the acid treatment of (a) is not performed, the electrode to which impurities are attached is subjected to heat treatment for (b), and the expected catalytic activity of the heat treatment of (b) decreases. After the impurities are removed by the acid treatment of (a), the electrode surface is clean. Therefore, by performing the heat treatment of (b),
This is because a sufficient catalytic activity effect is exhibited.

The acid treatment (a) and the heat treatment (b) as described above
In a single stage of the oxygen sensor element, the measurement may be performed on the entire element, or may be performed only on the porous electrode portion of such an element. In the state of an assembly (finished product) assembled as a sensor, it may be performed at all only for the electrode portion (porous electrode) exposed to the gas to be measured of the oxygen sensor element constituting the sensor. In addition, as shown in FIG. 1, the acid treatment (a) and the heat treatment (b) are performed by the electrode (4) exposed to the gas to be measured of the oxygen sensor element.
When the porous protective layer (8) is provided thereon, such an acid treatment (a) or a heat treatment (b) is performed through the porous protective layer (8).

Incidentally, the acid treatment (a) is generally carried out by immersing the oxygen sensor element in a suitable acid solution, and the acid for such acid treatment is phosphoric acid. , Hydrofluoric acid, borofluoric acid, hydrochloric acid, nitric acid, aqua regia and the like are appropriately selected and used, and among them, hydrofluoric acid treatment or borofluoric acid treatment is most desirable. The concentration of the acid used in such an acid treatment is appropriately selected at a concentration suitable for removing impurities covering the electrode surface. For example, in the case of hydrofluoric acid, Will be used in the concentration range of 0.05 to 40% by weight, preferably 0.1 to 10% by weight. When the concentration of hydrofluoric acid is lower than 0.05% by weight, it is difficult to remove impurities around the electrode. On the other hand, when the concentration exceeds 40% by weight, hydrofluoric acid is also contained in the solid electrolyte itself. This is because they react and cause deterioration of the solid electrolyte body.

Further, the treatment temperature at the time of the acid treatment is also appropriately selected according to the kind of the acid in order to sufficiently exert the treatment effect. It is particularly preferable to maintain the temperature at 30 to 50 ° C. in order to stabilize the effect of removing impurities of hydrofluoric acid. In addition, the oxygen sensor element which has been subjected to an acid treatment such as hydrofluoric acid may be subjected to a known cleaning process such as sufficiently washing the element subjected to such an acid treatment with running water or ultrasonic cleaning. Is preferably carried out appropriately, but especially when hydrofluoric acid treatment is performed, the oxygen is dissolved by a solution of an alkaline earth metal salt such as Mg (NO ₃ ) ₂ or Ca (NO ₃ ) _2. By treating the sensor element, fluoride ions are fixed and the effect of removing impurities of hydrofluoric acid can be completely cut off, so that the adverse effect due to the residual hydrofluoric acid can be avoided.

The heat treatment (b) as described above, which is performed on the porous cermet electrode according to the present invention, is performed in an atmosphere of at least one of a reducing gas and an unsaturated hydrocarbon. Become. The temperature of this heat treatment is preferably in the range of 300 to 1100 ° C.,
A range of 950 ° C. is more preferred. Further, the temperature during the treatment does not need to be constant, and a thermal cycle between 300 and 1100 ° C. does not matter at all. This heat treatment temperature is 30
When the temperature is lower than 0 ° C., the above-described activation effect of the oxygen sensor element is weak, and there is a possibility that the variation in the obtained product is large.Also, when the heat treatment temperature exceeds 1100 ° C., the There is also a problem that sintering and wear rapidly progress. Further, the heat treatment time is preferably about 1 to 48 hours, more preferably 5 to 16 hours. When the heat treatment time is less than 1 hour, the activation effect is weak,
If the resulting product has large variations and the heat treatment time exceeds 48 hours, the metal used for the electrodes will be significantly sintered and worn.

As the reducing gas used in the heat treatment (b), H ₂ , CO, or a mixed gas thereof is used as N ₂ , CO
₂ or a gas obtained by incompletely burning natural gas, LPG, or the like, which is diluted with a mixed gas thereof, or H ₂ is 10 to 40% by volume, and N ₂ is 10 ~ 40
It is preferable to use a mixed gas having a composition in the range of 0.1% to 5% by volume, CO _{2 in} the range of 0.1 to 5% by volume, and CO in the range of 5 to 40% by volume.

The unsaturated hydrocarbons, acetylene in a gaseous state at room temperature, ethylene, propylene, butylene or the like is used, the neutral gas such as H ₂ and He as balance gas, 100 ppm of such unsaturated hydrocarbons The target oxygen sensor element is subjected to a heat treatment in an atmosphere containing a concentration in the range of 10 to 10% by volume, preferably 500 ppm to 5% by volume. When the concentration of the unsaturated hydrocarbon is less than 100 ppm, the above-described activation effect of the oxygen sensor element is weak, the variation in the obtained product is large, and when it exceeds 10% by volume, carbon during the heat treatment is reduced. This is because they may precipitate and deteriorate the electrode.

Note that the reducing gas atmosphere or the unsaturated hydrocarbon atmosphere employed during the heat treatment as described above does not need to be constant.For example, in a reducing gas atmosphere, O ₂ , air, or the like is periodically introduced. By doing so, an atmosphere cycle for alternately realizing a rich atmosphere and a lean atmosphere can be performed.

Although the heat treatment in the reducing gas atmosphere and the heat treatment in the unsaturated hydrocarbon atmosphere as described above can achieve the desired effects even if performed independently, it is particularly necessary to perform the two processes together. Is more effective. These two treatments can be performed separately in the production of the oxygen sensor element.However, by using a mixed gas containing a predetermined amount of unsaturated hydrocarbon in a reducing gas as a heat treatment atmosphere, It is efficient to do it at the same time. In this case, the amount of the unsaturated hydrocarbon present in the reducing gas is 100 ppm to 10% by volume, preferably 500 ppm to 5% by volume, as in the case of the unsaturated hydrocarbon alone.
Is set within the range.

(Examples) Hereinafter, in order to clarify the present invention more specifically, representative examples according to the present invention will be described. However, the present invention should not be construed as being limited by the description of such examples. Not that, of course.

In addition, the present invention can be carried out in various modes in addition to the above-described specific description of the present invention and the following examples. It should be understood that any implementation in various aspects based on knowledge falls within the scope of the present invention.

Example 1 100 of a mixture consisting of 4 mol% of Y ₂ O ₃ and 96 mol% of ZrO ₂
ZrO ₂ raw material powder obtained by adding 3 parts by weight of clay as a sintering aid to parts by weight is dry-pulverized.
10 parts by weight of polyvinyl butyral as a binder, 5 parts by weight of DOP as a plasticizer, and 100 parts by weight of a toluene solvent were added to 0 parts by weight to prepare a slurry, and the viscosity was adjusted. Was. Then from the resulting slurry, using a doctor blade, thickness:
A 0.4 mm zirconia green sheet for a solid electrolyte body was formed.

On the other hand, by the same method as the above-mentioned method for producing a green sheet, 8 mol% of Y ₂ O ₃ containing 15% by weight of the sublimable powder (theobromine) -92 mol% of ZrO _{2 and} a thickness of ZrO ₂ raw material of 0.2% A green sheet for a porous protective layer having a thickness of mm was formed.

Then, in order to obtain an oxygen sensor element having the structure shown in FIG. 1, the above-mentioned zirconia green sheet for a solid electrolyte body is used, and predetermined portions on the upper and lower surfaces thereof are made of only ZrO ₂ to which no stabilizer is added. Using a cermet paste consisting of 15% by weight of ZrO ₂ powder having various average particle diameters shown in Table 1 and 85% by weight of Pt powder as a catalyst metal, the measurement electrodes (4 ) And a reference electrode (6) were formed to obtain various green sheets for electrode-formed solid electrolyte bodies. Using the zirconia green sheets for a solid electrolyte body, a spacer (12) for forming an air passage (10) and a plate-like lid member (14) were prepared.

Furthermore, a zirconia green sheet for a solid electrolyte similar to that described above was used as a heater green sheet for providing an airtight layer (22), and MgO: 2 parts by weight with respect to 100 parts by weight of Al ₂ O _3. With respect to 100 parts by weight of the Al ₂ O ₃ powder raw material, polyvinyl butyral: 15 parts by weight, DOP: 10
Parts by weight and butyl carbitol: screen-printing the electrical insulating layer (20) using an alumina paste containing 20 parts by weight, and forming a P on the electrical insulating layer (20).
t: 90 parts by weight% -AlO _3: Using 10% by weight consists of cermet paste to form the heater element (16), as further buried the heater element (16), an electrically insulating layer (18) Was prepared using a cermet paste similar to that for the electric insulating layer (20).

Thereafter, the four sheets prepared as described above (the green sheet for the electrode-formed solid electrolyte body, the green sheet for the spacer, the green sheet for the plate-shaped lid member, and the fourth sheet) and the green sheet for the porous protective layer ( 8) were laminated and integrated, and then simultaneously and integrally fired at a temperature of 1400 ° C. for 3 hours to produce various oxygen sensor elements having a structure as shown in FIG.

Example 2 Using various fired elements obtained in the same manner as in Example 1, they were immersed in a 0.5% aqueous solution of hydrofluoric acid heated to 30 ° C. for 10 minutes, and then sufficiently washed with running water. . Next, the element subjected to such a treatment was left un-dried for 10 minutes.
% Ca (NO ₃ ) ₂ aqueous solution and decompressed to obtain Ca (NO ₃ ) ₂
After sufficiently penetrating the aqueous solution into the porous protective layer of the element, the element was subjected to ultrasonic cleaning, and then dried to produce an oxygen-treated oxygen sensor element.

Example 3 Using various fired devices obtained in the same manner as in Example 1, such devices (measuring electrodes) were subjected to heat treatment in a reducing gas and unsaturated hydrocarbon.

That is, N ₂ , H ₂ and CO ₂ gas are fed into the catalytic converter,
Therefore, after converting to a volume ratio of CO / CO ₂ / H ₂ / N ₂ = 10/2/50/38 to produce a base gas, 100 volumes of this base gas are converted into unsaturated hydrocarbons. 0.5 of propylene
It is added at a volume ratio and supplied into a reduction treatment furnace.
By performing the heat treatment at the above temperature for 5 hours, an oxygen sensor element subjected to the intended heat treatment was produced.

Example 4 By using various acid-treated elements obtained in the same manner as in Example 2 and performing a heat treatment in a reducing gas similar to that in Example 3, acid treatment and heat treatment were performed. The manufactured oxygen sensor element was manufactured.

Examples 5 to 8 The ZrO ₂ raw material powder (4 mol% Y ₂ O ₃ -96 mol% ZrO ₂ ) constituting the zirconia green sheet for a solid electrolyte body in Example 1 was used, and the average particle size was as shown in Table 2 below. 15% by weight and 85% by weight of Pt (catalytic metal) powder
An oxygen sensor element was produced by simultaneous firing operation in the same manner as in Example 1 except that electrodes (4, 6) were formed by a screen printing method using a cermet paste made of

Then, for the obtained fired element, Examples 2 to
In accordance with the same method as in Example 4, a heat treatment in a hydrofluoric acid treatment and / or a reducing atmosphere was performed to produce an oxygen sensor element subjected to a desired acid treatment and / or heat treatment.

Of the oxygen sensor elements obtained in this way, those that had not been subjected to any post-treatment were used as the elements of Example 5, while only the acid treatment, only the heat treatment, and those subjected to both of them were: The devices of Examples 6, 7, and 8 were used.

Evaluation of low-temperature operability and responsiveness Using various oxygen sensor elements manufactured in each of the above-described examples, as in the conventional case, an oxygen sensor was assembled and each low-temperature operability and responsiveness were evaluated. .

The low-temperature operability is evaluated by applying a voltage to each sensor element to generate heat by applying a voltage to a heater element built in the sensor element to maintain CO, H ₂ ,
The N ₂ and the excess air ratio in the measurement gas consisting of air is changed variously to measure therebetween sensor electromotive force, the air excess ratio at which the sensor electromotive force becomes 0.45 V: as lambda _D, low temperature operation It was a measure of gender.

In addition, the evaluation of the responsiveness is performed by driving the actual engine at a rotational speed of 1150 rpm while maintaining the respective oxygen sensor elements at a predetermined temperature by generating heat by supplying a voltage to the built-in heater element, when the feedback control in such a sensor element, the feedback frequency sensor electromotive force reaches between 0.30~0.60V within a certain time period: seeking F _LC, and that to the response of the measure.

The evaluation results of the low-temperature operability and responsiveness of the oxygen sensor elements obtained in Examples 1 to 8 are shown in Tables 3 and 4 below and FIGS. 2 to 7, respectively. FIG. 2 and FIG.
The evaluation results of the sensor element No. 3 are shown, and the evaluation results of the low-temperature operability shown in FIGS. 4 and 5 show the electrode raw materials for the various oxygen sensor elements obtained in Example 4. The graph shows the dependence of the particle size of the monoclinic ZrO ₂ powder and the particle size of the zirconia particles in the fired electrode. Furthermore, the operability evaluation results shown in FIGS. 6 and 7 show the particle size of the electrode raw material: zirconia powder and the excess air ratio (λ _D ) for the various oxygen sensor elements obtained in Example 8. 4 shows the relationship with the particle size of zirconia particles in a fired electrode.

As is clear from the results shown in Tables 3 and 4 and FIGS. 2 to 7, the porosity obtained by distributing the zirconia particles in the electrode exposed to the gas to be measured in a particle size of 2 μm or less according to the present invention. It is understood that both the low-temperature operability and the responsiveness of the oxygen sensor element can be greatly improved by using a high quality cermet electrode and by applying a predetermined acid treatment or heat treatment to such a porous electrode. You.

(Effects of the Invention) As is clear from the above description, in the oxygen sensor element according to the present invention, the electrode exposed to the gas to be measured has a porous cermet electrode structure having a large surface area, and the catalytic activity is enhanced. Therefore, it is possible to produce an oxygen sensor element excellent in low-temperature operation and responsiveness according to the method of the present invention. It is possible to further improve characteristics such as low-temperature operability and responsiveness of the oxygen sensor element, and to advantageously detect a low-temperature gas to be measured.

[Brief description of the drawings]

FIG. 1 is an exploded perspective view showing an example of a typical structure of an oxygen sensor element to which the present invention is applied, and FIGS. 2 and 4 to 7 are each manufactured in each embodiment. FIG. 3 is a graph showing the relationship between the sensor element temperature and the excess air ratio for the oxygen sensor element, and FIG. 3 further shows the sensor element temperature, the feedback frequency, 6 is a graph showing the relationship of. 2: Solid electrolyte, 4: Measurement electrode 6: Reference electrode, 8: Porous protective layer 10: Air passage, 12: Spacer 14: Plate lid member, 16: Heater element 18, 20: Electrical insulating layer 22: Airtight layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-31059 (JP, A) JP-A-1-184457 (JP, A) JP-A-1-176937 (JP, A) 125084 (JP, A) JP-A-61-120055 (JP, A) JP-A-63-82356 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/409

Claims (4)

(57) [Claims] When manufacturing an oxygen sensor element in which an oxygen ion conductive zirconia solid electrolyte body supports at least a pair of electrodes, at least one of the at least one pair of electrodes exposed to a gas to be measured is used. After applying the electrode forming material prepared from a catalyst metal powder and a zirconia powder having a particle diameter of 0.2 to 2.5 μm comprising monoclinic ZrO ₂ to the solid electrolyte body, by firing, the porous cermet A step of forming an electrode, an acid treatment of the porous electrode formed by the firing, and a heat treatment of the porous electrode formed by the firing in a reducing gas and / or unsaturated hydrocarbon atmosphere. A method for manufacturing an oxygen sensor element, comprising: 2. A method of manufacturing an oxygen sensor element comprising a zirconia solid electrolyte body having oxygen ion conductivity and supporting at least a pair of electrodes, wherein at least one of the at least one pair of electrodes exposed to a gas to be measured is used. After applying the electrode forming material prepared from a catalyst metal powder and a zirconia-based powder having a particle diameter of 0.2 to 2.0 μm comprising partially stabilized zirconia to the solid electrolyte body, by firing, the porous Forming a cermet electrode, acid-treating the porous electrode formed by the firing, and heat-treating the porous electrode formed by the firing in a reducing gas and / or unsaturated hydrocarbon atmosphere. A method for manufacturing an oxygen sensor element, comprising: 3. The method according to claim 1, wherein the zirconia powder comprises CaO, Y ₂ O ₃ and Yb.
_3. The production method according to claim 1, wherein the production method comprises partially stabilized zirconia formed by adding at least one of ₂ O ₃ at a ratio of 4 mol% or less. 4. The method according to claim 1, wherein the heat treatment in the reducing gas atmosphere and the heat treatment in the unsaturated hydrocarbon atmosphere are performed simultaneously.