JPH02150758A - Oxygen sensor element - Google Patents

Abstract

PURPOSE:To exactly sense a theoretical air-fuel ratio in a low-temp. gas by constituting a measuring electrode of cermet consisting of a catalyst metal and ceramics and coating the electrode with a porous layer introduced with a Pt group at a prescribed ratio. CONSTITUTION:An air passage 12 the front end of which is blinded is provided in a body consisting of a solid electrolyte having an oxygen ion conductivity, such as zirconia. This passage 12 is opened at the base end of an element body 10. A reference electrode 14 and the cermet measuring electrode 16 face each other on the surface in the passage and the measuring electrode 16 is coated by the prescribed thickness with the porous protective layer 18 consisting of alumina. Air is introduced through the passage 12 and is brought into contact as a reference gas of a prescribed standard O2 concn. The electrodes 14, 16 are heated by the embedded heater 22 of the body 10 to a prescribed temp. The O2 concn. is known from the electromotive force based on a difference in the O2 concn. in the gas in contact with the electrodes 14, 16. The electrode 16 is constituted of the cermet consisting of the catalyst metal and the ceramics. One or plurality of Pt, Pd, etc., are introduced at 10 to 1,500ppm ratio into the protective layer 18.

Description

Detailed Description of the Invention (Technical Field) The present invention mainly relates to an oxygen sensor element for measuring the amount of oxygen contained in exhaust gas from internal combustion engines of automobiles, combustion equipment in industrial furnaces, etc. and an oxygen sensor element with improved electromotive force characteristics.

(Background technology) Conventionally, oxygen ion-conducting solid electrolytes such as zirconia porcelain have been used to measure gases emitted from internal combustion engines in automobiles and combustion equipment in industrial furnaces, etc., using the principle of oxygen concentration batteries. detects the oxygen concentration in the exhaust gas of
It is known to control the air-fuel ratio or combustion state of such internal combustion engines.

In this type of oxygen sensor, which is an oxygen concentration detector, the sensor element is made of a catalytic metal such as platinum, A predetermined electrode containing the same is provided, and the inner electrode is used as a reference electrode that is exposed to a reference gas such as air with a reference oxygen concentration, while the outer electrode is exposed to exhaust gas that is the gas to be measured and used as a measurement electrode. The oxygen concentration in the exhaust gas is measured by detecting an electromotive force based on the difference in oxygen concentration between the reference electrode and the measurement electrode.

In addition, in such an oxygen sensor element, the measurement electrode, which is the outer surface electrode, may be worn out or damaged by the action of the high temperature exhaust gas, which is the gas to be measured, resulting in problems such as deterioration of the sensor function. In order to protect the measurement electrode from the occurrence of such problems, a porous protective coating layer such as spinel is formed on the measurement electrode to a predetermined thickness by plasma spraying or the like.

By the way, this type of oxygen sensor element is required to accurately sense the stoichiometric air-fuel ratio point over a wide temperature range due to its intended use, but the catalytic ability of the electrode decreases at low temperatures. Therefore, the electromotive force characteristic curve, so-called λ
The curve shifts to the normal (oxygen-excess atmosphere) side,
Therefore, with conventional sensors, the temperature of the gas to be measured is 400°
When the temperature is below C, it becomes difficult to accurately sense the stoichiometric air-fuel ratio point.

For this reason, Japanese Patent Application Laid-Open No. 54-41794 discloses a catalyst prepared by dispersing a platinum metal catalyst in a proportion of 0.1 to 5% by weight on a measurement electrode composed of a porous metal electrode such as platinum. A technique has been revealed to prevent the deviation of the electromotive force characteristic curve by providing a layer, but in this case, in addition to the catalytic metal such as platinum used as the measurement electrode, a relatively large amount of the same type of metal is used. Because of this necessity, in addition to inevitably causing an increase in cost, there is also the inherent problem that the responsiveness of the sensor is significantly reduced.

On the other hand, some of the electrodes of oxygen sensor elements are stabilized by mixing catalytic metal powder and ceramic powder in order to improve their durability under severe usage conditions such as long-term or extremely high temperatures. Alternatively, cermet electrodes are known that are baked onto a sensor element (element body) made of partially stabilized zirconia or the like, or are baked integrally with a base.

However, such cermet electrodes are generally heated to at least 1000°C to improve their durability.
1. Since baking is performed at a high temperature, preferably 1300° C. or higher, or the sensor body is baked integrally with the sensor body, the catalytic activity as an electrode becomes extremely poor, and the range of its use is limited.

In addition, in order to overcome the drawbacks of sensor electrodes such as cermet electrodes and electrodes other than cermet electrodes, such as plated electrodes, oxygen sensors have been devised in which the electrode portion is heated with a heater. However, no matter how heated with a heater, the side of such a sensor exposed to the gas to be measured (exhaust gas) is cooled by the gas when the gas salt is low, reducing the catalytic activity of the electrode. , deviations are likely to occur in the electromotive force characteristic curve.

In particular, in recent years, exhaust gas regulations have become stricter, and due to constraints such as sensor elements that operate even when vehicles are idling, and sensor mounting positions, it is now possible to accurately control the air-fuel ratio even in the low-temperature area behind the exhaust pipe. However, with conventional oxygen sensor elements, it was impossible to obtain accurate electromotive force characteristic curves in exhaust gas at low temperatures below 400°C. It is.

(Problem to be solved) The present invention has been made to solve all of the problems in view of the above circumstances, and the main problem to be solved is to An object of the present invention is to obtain an oxygen sensor element capable of accurately sensing a stoichiometric air-fuel ratio point without impairing responsiveness.

(Solution Means) In order to achieve the above objects, the present invention has the following features:
An oxygen ion conductive solid electrolyte, a reference electrode provided in contact with the solid electrolyte and exposed to a reference gas having a predetermined reference oxygen concentration, and a reference electrode provided in contact with the solid electrolyte and exposed to a gas to be measured. In an oxygen sensor element comprising at least a measuring electrode and a porous coating layer formed on the measuring electrode to cover and protect the measuring electrode, the measuring electrode is made of a cermet made of a catalytic metal and ceramics. 10 to 1500 p of one or more platinum group metals in the porous coating layer.
The gist of this is that it was introduced at a rate of pm.

(Function/Effect) As described above, the present invention provides a measurement electrode provided on the surface of a predetermined oxygen ion conductive solid electrolyte.
It is composed of a cermet made of a catalytic metal such as platinum and a ceramic such as zirconia, and the responsiveness is impaired by introducing an appropriate amount of one or more platinum group metals into the porous coating layer covering the measurement electrode. It was completed based on the discovery that the stoichiometric air-fuel ratio point can be accurately detected in low-temperature gas without any

Here, the appropriate amount of platinum group metal introduced into the porous coating layer is 10 to 1500 ppm based on the weight of the porous coating layer in the total amount,
Desirably less than 11,000 pp, more preferably 10 to
It is in the range of 500 ppm. If the ratio of this introduced amount is less than 10 ppm, the electromotive force characteristics of the oxygen sensor element will become extremely unstable, and at about 2 ppm,
When oxygen sensor elements are manufactured in large quantities, the performance of each oxygen sensor element varies greatly. That is, 10 Ppm
Only in the above-mentioned state will the quality become stable. Furthermore, when the total amount of platinum group metals introduced exceeds 1500 ppm, although the electromotive force characteristics are stabilized, the responsiveness is significantly reduced, causing undesirable problems in terms of air-fuel ratio control. Note that in order to have responsiveness comparable to that of a normal oxygen sensor element and to obtain stable electromotive force characteristics at low temperatures, it is preferable to keep it less than 11,000 ppm, preferably less than 500 ppm.

In short, the present invention uses a cermet-type measurement electrode and introduces a small amount of a specific platinum group metal into the porous coating layer covering the electrode, thereby improving the oxygen sensor element without reducing the response. It has been able to significantly improve low-temperature operability, and even when the amount of platinum group metal used is reduced, the effective electromotive force characteristic curve can be stabilized even when the gas salt content of the gas to be measured is low. However, these characteristics can also be obtained by using an oxygen sensor structure that is equipped with a heater means that heats at least the measurement electrode disposed portion of the sensor element. In addition to improved performance, it becomes possible to obtain accurate electromotive force characteristic curves over a wider range of temperatures, thereby further increasing the effects of the present invention.

(Specific Structure) By the way, FIG. 1 shows an example of a typical structure of the oxygen sensor element according to the present invention.

That is, the oxygen sensor element shown in FIG. 1 is in the form of a plate formed by a known lamination method, and is made of an oxygen ion conductive solid electrolyte such as zirconia and has a narrow width of a predetermined length. An air passage 12 whose tip end is a dead end is provided in the plate-shaped element body 10. Note that this air passage 12 opens at the base end of the element body 1O and is communicated with the atmosphere. A reference electrode 14 is provided on the inner surface of the element body 10 exposed in the air passage 12, and the air introduced through the air passage 12 has a predetermined reference oxygen level with respect to the reference electrode 14. It is designed to be brought into contact as a reference gas for concentration. Further, on the outer surface of the element main body 10 so as to face the reference electrode 14,
A measurement electrode 16 made of a cermet electrode is provided. A porous coating layer 18 made of alumina or the like is provided on the measurement electrode 16 to cover it and has a predetermined thickness. A measuring gas can be guided and brought into contact with the measuring electrode 16.

Note that a heater element 22 embedded in an electrically insulating layer 20 made of alumina or the like is integrally provided in the element body 1O, and when the heater element 22 is energized from the outside, the heater element 22 is By generating heat, the portion where the electrodes 14 and 16 of the oxygen sensor element are buried is heated to a predetermined temperature.

Therefore, in an oxygen sensor element having such a structure,
As is well known, the oxygen concentration in the atmosphere (measured gas) brought into contact with the measurement electrode 16 and the reference electrode 14
The oxygen concentration in the target gas to be measured is detected according to the electromotive force generated based on the difference in oxygen concentration in the atmosphere (reference gas) that is brought into contact with the gas. As described above, in the present invention, cermet is used as the measurement electrode 16, and a specific small amount of a platinum group metal is introduced into the porous protective layer 18, so that the measurement can be performed with good responsiveness and accuracy. This made it possible to obtain an electromotive force characteristic curve.

Here, as the oxygen ion conductive solid electrolyte constituting the oxygen sensor element, various known ones can be mentioned, but in the present invention, in particular, zirconia is added with a predetermined stabilizer, such as A stabilized or partially stabilized zirconia material containing O, ), calcia (Cab), magnesia (MgO), ytterbia (YbzOs), etc. is preferably used.

As is well known, such solid electrolyte materials also contain certain sintering aids, such as clay such as kaolin, Si
Ch, Algos*Fezcl+, etc. will be blended as appropriate.

Then, a molded body that provides the element body of the oxygen sensor element in a predetermined shape is formed using a material selected from these solid electrolyte materials. Well-known methods such as pressure molding methods such as the method and lamination methods such as the thick film method are adopted, and thereby the element body (substrate) in the shape of a cylinder with a bottom or a plate shape, which is the main body of the oxygen sensor element, is formed. A molded body is formed.

Next, such a molded body is subjected to a calcination operation at a temperature lower than the sintering temperature, if necessary, and then sintered according to a known normal sintering operation. Prior to or after the firing, at least a measurement electrode exposed to the gas to be measured and a reference electrode exposed to the reference gas having a predetermined reference oxygen concentration are formed on the surface of the molded body in the same manner as in the past. be.

Note that these electrodes are made of platinum group metals known as catalytic metals, such as platinum, ruthenium, and osmium.

It is formed as a thin film electrode made of a conductive material mainly made of iridium, rhodium, palladium, etc., or platinum group metals, but platinum is generally used as the catalyst metal. Also, platinum to which approximately 1 to 30% of other platinum group metals such as rhodium and palladium are added is used.

Among these electrodes, the reference electrode can be formed using the conventionally known plating method, sputtering method, thermal decomposition of a salt of the electrode metal (catalyst metal), or forming a cermet paste of the electrode metal and ceramics. Select and adopt as appropriate from various methods such as a paste baking method in which the cermet paste is baked onto the body surface, and a simultaneous cermet paste baking method in which the cermet paste is printed or applied on the element body and then fired together with the element body. However, since the measuring electrode needs to be constructed as a cermet electrode according to the present invention, among the above electrode forming methods, either the paste baking method or the cermet paste co-firing method is adopted. The Rukoto.

Furthermore, when forming these electrodes, etching the surface of the molded body (solid electrolyte) as a pretreatment such as plating or sputtering is advantageous in improving the adhesion and performance of the electrodes formed there. A cermet paste that provides a cermet electrode such as an electrode is a mixture of the above-mentioned platinum-based catalyst metal powder (platinum powder or a mixed powder of it with other metals, etc.) and the above-mentioned oxygen ion conductive solid electrolyte material powder. is advantageously used, but it is also possible to use other ceramic powders such as alumina and calcia in place of such solid electrolyte powders. And such cermet paste is
After printing or brushing on a predetermined position on the surface of the molded body, the adhesion force of the electrode to the base material can be maximized by integrally firing the electrode together with the molded body or by laminating it together with the molded body.

After a predetermined electrode is attached to a predetermined element body as described above, a porous film with a predetermined thickness is placed on the measurement electrode that will be exposed to the gas to be measured. A ceramic coating layer is formed. This ceramic coating layer can also be formed according to various known methods, such as spinel (Al1
．． 03Mg0), zirconia, alumina, etc., by spraying them by plasma spraying or flame spraying to form a coating layer of a predetermined thickness, or printing it on the electrode as a ceramic powder slurry. Alternatively, it is formed by dipping into such a slurry and then baking it, or by applying it on a cermet and firing it together. In addition, from the viewpoint of durability, the porous coating layer formed by integral firing uses the solid electrolyte pulverized powder as described above, mixes it with a machine binder, and forms it into a sheet. It is preferable that it be formed using

In addition, the structure of such a porous coating layer is not limited to a single layer, but may also be a multilayer structure of two or more layers (
Furthermore, the porosity (volume ratio of pores in the volume of the coating layer) is appropriately selected, but is generally about 10 to 60%. Barefoot,
If this porosity is thicker than 60%, the gas permeability of the porous coating layer will be good and the responsiveness will improve, but on the other hand,
This is because the durability against poisoning, etc. decreases, and if the porosity is less than 10%, while the durability against poisoning, etc. is good, the responsiveness significantly decreases. It is from.

In the present invention, a predetermined amount of platinum group metal is further contained in the formed porous coating layer, and methods for this purpose include a dipping method, a slurry method, a vapor deposition method, etc. There are various methods.

More specifically, in the immersion method, a sensor element coated with a porous coating layer is immersed in an aqueous solution of a platinum group metal compound such as HzP t C1h or RhC1x, or a mixture thereof in a container capable of reducing pressure. The measuring electrode portion is immersed and the pressure is reduced to introduce the solution into the pores of the coating layer, followed by drying, followed by reduction treatment at a relatively low temperature, for example, 200 to 500°C. , in a method of dispersing a platinum group metal within a porous coating layer by metallizing a platinum group metal compound. Note that the reduction method includes heat treatment in a reducing gas atmosphere such as H, CO, etc., or immersion in an alkaline aqueous solution such as NaBHa. This dipping method is most effective in finely and uniformly dispersing the platinum group metal within the porous coating layer.

In addition, in the slurry method, a suitable compound of a platinum group metal is added to T-A/! □It is carried out by mixing the slurry with 03, etc., and applying it to the surface of the measurement electrode of the sensor element or the porous coating layer covering the measurement electrode, and heating and baking it in a reducing atmosphere. Become.

Furthermore, the vapor deposition method coats the surface of the porous coating layer provided on the measurement electrode of the sensor element.
This is carried out by depositing a platinum group metal by ion sputtering.

Note that the platinum group metals introduced into this porous coating layer include platinum, ruthenium, osmium, iridium, rhodium, palladium, etc., and one or more of these metals are contained in an amount of 10 to 1500 ppm as described above. In particular, in the present invention, combinations of platinum and other platinum group metals,
Among these, a combination of platinum and rhodium is advantageously used. In addition, in this combination of platinum and rhodium, the Pt/Rh ratio (weight) is 10015 to 10.
It is preferably used at a ratio within the range of 15.

In addition, the platinum group metal introduced into the porous coating layer must be present at least on the surface of the coating layer that comes into contact with the gas, but it must be uniformly dispersed and contained throughout the coating layer. However, it is preferable that the catalyst layer be formed on the surface of such a coating layer that is brought into contact with gas to a predetermined thickness and present unevenly on the surface of such a coating layer. No problem. Furthermore, in addition to the platinum group metal, this porous coating layer contains an oxide of a rare earth element such as Cent in an amount of 5 to 5 oxides based on the weight of the platinum group metal.
It is also possible to add and introduce about 0%.

By the way, various known methods are adopted to determine the amount of platinum group metal in the porous coating layer introduced in this way. For example, in the case of an aqueous solution impregnation method, the porosity of the porous coating layer , is determined according to the following formula from the platinum group metal concentration in the impregnating solution, the density of the porous coating layer, etc.

pxv,=v.

v, xc, = w.

(1-P) x vp x Dp = WP W, /Wp = 10 to 1500 ppm, where P: porosity v of the porous coating layer; volume v of the porous coating layer: platinum group metal in the porous coating layer Volume of solution C1: Concentration of platinum group metal in solution W, : Weight of platinum group metal applied to porous coating layer, : Density of porous coating layer W, : Weight of porous coating layer Also, slurry In the case of the coating method, it is determined from the weight of the granular ceramics added to the slurry and the weight of the platinum group metal, and the amount of platinum group metal introduced is determined by various known chemical analysis methods or instrumental analysis methods. This will be requested as appropriate. Note that if the platinum group metal in the porous coating layer and the catalyst metal constituting the measurement electrode are the same, a method such as scraping off only the porous coating layer and analyzing it is adopted.

The measurement electrode thus obtained is a cermet electrode,
Further, an oxygen sensor element in which a predetermined small amount of a platinum group metal is introduced into a porous coating layer formed thereon is capable of functioning as a measuring electrode under the action of such a small amount of platinum group metal. The cermet electrode comes into contact with the gas to be measured, without significantly reducing the response.
It is possible to obtain an accurate electromotive force characteristic curve even at low temperatures, and as an oxygen sensor element with effective characteristics, it can be advantageously applied to the target oxygen sensor. It is.

(Example) In order to clarify the present invention more specifically, typical examples according to the present invention will be shown below. It goes without saying that this is not something that can be done.

Furthermore, the present invention can be implemented in various embodiments in addition to the detailed description of the present invention described above and the following examples, and as long as they do not depart from the spirit of the present invention,
It should be understood that various embodiments based on the knowledge of those skilled in the art fall within the scope of the present invention.

First, using a solid electrolyte material made by mixing 96 mol% zirconia and 4 mol% ittria and further adding 2% by weight of clay as a sintering aid, an element shape as shown in Fig. 1 was prepared. An oxygen sensor element with a built-in heater was manufactured according to a known lamination method. Note that the porous protective layer (18) is a sheet of porous zirconia material mixed with a burnt-out material, and a sheet of porous zirconia material mixed with a burnt-out material, and a sheet of porous zirconia material mixed with the element body (18) formed of the solid electrolyte material.
The reference electrode (14) and the measurement electrode (16) also contain Pt: 85% by weight and the above-mentioned solid. Electrolyte material: Using a cermet paste having a proportion of 15% by weight, it was partially applied to the tip of the element within a range of 1 to 8.5 mm, and was formed by firing at the same time as the element body (10).

Next, a length of about 12 mm from the tip of the oxygen sensor element obtained in this way was exposed to various concentrations of (Hz P
t Cf b + Rh Cl s ) immersed in an aqueous solution,
Impregnation is carried out under reduced pressure for 10 minutes, then at 80°
After drying at C for 30 minutes, odor was removed in H2 atmosphere.
By performing heat treatment at 00°C for 1 hour, oxygen sensor elements with different amounts of platinum group metal impregnated were obtained. In addition, (8
2P t CIlb +Rh CI! , +) The concentrations of Pt and Rh in the aqueous solution and the amount of platinum group metal impregnated into the porous protective layer (18) are as shown in Table 1 below. Further, in both cases, the impregnation ratio of PL and Rh (PL/Rh) was 10/1.

Table 1 Various sensor elements into which platinum and rhodium were introduced were assembled into a metal case like a normal oxygen sensor, and their respective performances were evaluated. This performance evaluation is
Using a model gas, the electromotive force characteristic curves of each sensor element at an element temperature of 400°C were determined, and the results are shown in FIG. As is clear from FIG. 2, when the amount of platinum group metal impregnated increases, especially when the amount exceeds 360 ppm, the electromotive force characteristics show approximately the same curve (rise) and are better than the theoretical value of the untreated sensor element. It is confirmed that the fuel ratio point is accurately sensed.

Also, the responsiveness of each sensor element, that is, when the gas to be measured is switched from a reducing gas (rich) to an oxidizing gas (lean), the electromotive force of the sensor element changes from 0.6 to 0.
The response time required to change to 3V was evaluated, and the results are shown in Table 2 below. As is clear from Table 2, the response time: TRL is
000 ppm, and from this it is understood that the response becomes worse as the amount of platinum group metal introduced increases. In Table 2, "" indicates the number of responses per unit time when the sensor is actually attached to the engine, and the larger the number, the faster the response. , when the amount of platinum group metal introduced is large (3000 ppm
), a decrease is observed.

Table 2

[Brief explanation of the drawing]

FIG. 1 is an explanatory cross-sectional view showing an example of a typical structure of an oxygen sensor element according to the present invention, and FIG. It is a graph showing an electromotive force characteristic curve. 10: Element body 12: Air passage 14: Reference electrode 16: Measurement electrode 18: Porous protective layer 20: Electrical insulation layer 22: Heater element 2 @ 1st WA A: Ft/Rh -10/1 3000? pn
+*2 pt/Rh-1'/1'/θρP
? ”” Pt/R/+ g ”/1 360 ppn+
A: h/Rj, -To/l 40PPm mouth 2 Car processing! 0.98 1.00 λ 1.02 1.04 [-] Procedural Dispute Paper (self-motivated) December 29, 1999 1. Indication of the case 1988 Patent layer No. 305067 2. Name of the invention Oxygen Sensor element 3, relationship with the person making the amendment Patent applicant name (406) Nippon Insulator Co., Ltd. 4, agent 6, content of amendment (1) Specification tube 8'11. Line 9: “Cause.”
After , insert the following text without starting a new line. ``However, this does not apply if the deterioration of responsiveness is not a problem.'' (2) Do not put a line break next to ``Tonaru,'' on the second line of page 18. Insert text. ``Also, it may be formed by mixing a predetermined amount of ceramic powder and platinum group metal powder with an organic binder and forming it into a sheet.'' (3) Same page 24 top descending~
"Substantially the same curve (rise)" in the first line of page 25 is corrected to "substantially the same curve (rise) as the good electromotive force characteristic when impregnated at 13,000 ppm." (1) Detailed description of the invention in the specification

Claims (1)

[Claims] An oxygen ion conductive solid electrolyte, a reference electrode provided in contact with the solid electrolyte and exposed to a reference gas having a predetermined reference oxygen concentration, and a reference electrode provided in contact with the solid electrolyte and exposed to a gas to be measured. In an oxygen sensor element comprising at least a measuring electrode and a porous coating layer formed on the measuring electrode to cover and protect the measuring electrode, the measuring electrode is made of a cermet made of a catalytic metal and ceramics. 10 to 1500 p of one or more platinum group metals in the porous coating layer.
An oxygen sensor element characterized in that oxygen is introduced at a rate of pm.