JPH0814565B2 - Oxygen sensor element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to exhaust gas control of an industrial combustion apparatus that burns various fuels and uses thermal energy, and oxygen emission at high temperatures such as petrochemical, steel and metallurgy. Oxygen sensor element for controlling oxygen concentration in related chemical process, metallurgical process, etc., especially by detecting oxygen concentration of exhaust gas of automobile, which has severe operating environment conditions, small tolerance of characteristic change and long life The present invention relates to an oxygen sensor element for air-fuel ratio control.

(Prior Art and Problems) An oxygen sensor element for air-fuel ratio control mainly comprises a solid electrolyte and a pair of electrodes (reference electrode and measurement electrode) provided on the inner and outer surfaces thereof, and a measurement electrode (outer electrode; contact with exhaust gas) In order to protect the exhaust gas from exhaust gas, it is common to coat it with a porous protective layer. However, in the case of an oxygen sensor element used for air-fuel ratio control of an engine of a vehicle, for example, when the exhaust gas temperature is low (for example, when the engine is started), the exhaust gas flow is disturbed due to the disturbance of the atmosphere of each cylinder of the engine. , So-called "chemical noise" phenomenon occurs. That is, the sensor output concentrates in the vicinity of 500 mV and the control becomes irregular. Therefore, it is conceivable to make the entire protective layer dense, but the response of the sensor is reduced, and the atmosphere during high temperature control shifts to the lean side (excess air ratio λ> 1), increasing the NO _X emission amount. Come here.

In view of such circumstances, an object of the present invention is to provide an oxygen sensor element capable of suppressing the occurrence of a chemical noise phenomenon as much as possible without deteriorating the sensor responsiveness and causing a λ point shift, and thereby achieving an accurate air-fuel ratio control. And

(Means for Solving Problems) Means for solving the above problems according to the present invention are as follows.

The oxygen sensor element comprises a reference electrode on one surface of a tubular solid electrolyte having a closed front end and an open rear end, and a measurement electrode on the front end of the other surface, and the measurement electrode is covered with a porous protective layer. The front and rear ends of the porous protective layer are located on the front and rear ends of the solid electrolyte, respectively,
A portion up to a distance 1 along the axial direction of the tubular solid electrolyte from the tip of the porous protective layer is defined as a specific tip portion of the porous protective layer, and the axial direction from the tip to the rear end of the porous protective layer. When the total length along the line is L, there is a relation expressed by L / 5 ≦ l ≦ L / 2, and the thickness of the specific tip portion is increased by increasing the thickness of the porous protective layer itself. An oxygen sensor element having an average thickness of at least 1.5 times the thickness of the porous protective layer other than the specific tip portion.

(Preferred Embodiment and Action) The solid electrolyte is a tube (test tube) having one end closed and the other end open, for example, ZrO ₂ with Y ₂ as a stabilizer.
It is preferable to use a material to which O ₃ , CaO, etc. are added. Both the reference electrode and the measurement electrode (layered) are made porous, and it is preferable to use Pt or a noble metal such as Pt containing Rh of about 2% or less. The thickness of the "specific tip portion" of the porous protective layer must be larger than the thickness of the portion other than the specific tip portion (hereinafter referred to as "rear portion"). When the axial length (l) of the specific tip portion is less than 1/5 of the length (L) of the porous protective layer formed from the tip of the element to the attachment portion of the element along the axial direction of the solid electrolyte element, chemical If the noise prevention becomes insufficient, and (l) exceeds 1/2 of (L), the sensor response will deteriorate and the atmosphere during control will tend to shift to the lean side. This is because sometimes the NO _X emission increases. The range of 1/3 to 2/5 is particularly preferable. In the present specification, when the porous protective layer is provided right below the mounting portion (expanded portion) of the oxygen sensor element, the mounting portion serving as the rear end reference of the axial length (L) is held. The part used for joining to the body (metal case)
Shortening or extension in the forward or rearward direction is allowed at the mounting part. In terms of specific dimensions, (L) should be 30 mm or less, preferably 27 mm or less.

The thickness of the specific tip portion is preferably about 3/2 to 2 times the thickness of the rear portion. In terms of specific dimensions, the thickness of the specific tip portion should be greater than the thickness of the rear portion by 50 μm or more, preferably 80 μm or more. The thickness of the rear part should be about 80 to 150 μm, preferably about 130 μm. Therefore, the thickness of the specific tip part should be about 200 ± 50 μm, especially about 200 ± 25 μm. It is preferable that the specific tip portion and the rear portion each have a uniform thickness over their entire area (including both the axial direction and the circumferential direction), but they may be different. For example, in the specific tip portion, the front side may be gradually thickened, and similarly, in the rear portion, the front side (specific tip portion side) may be gradually thickened. Furthermore,
The axial length of the porous protective layer may be gradually increased from the rear end toward the front end. However, in this case, it is necessary that the average thickness of the specific tip portion is larger than the average thickness of the rear portion, especially 50 μm. When the element is attached to the holder, the axial length (l) of the specific tip portion and the element portion protruding from the tip surface of the holder (the portion that directly contacts the exhaust gas as the gas to be detected) The relationship with the axial length (L ') of is preferably 1 / 2L'≤l≤9 / 10L'.

The porous protective layer is made of ceramics such as Al ₂ O ₃ , spinel, B
It may be composed of eO, ZrO _2, etc., and spinel is particularly preferable. The porosity is preferably about 5 to 20%. Further, the protective layer may have a catalytic activity enhanced by containing a noble metal as a catalyst, for example, a platinum group element, dispersed therein. This catalyst can be dispersed uniformly or non-uniformly throughout the protective layer. For example, it is possible to increase the content rate of the noble metal in the specific tip portion rather than in the rear portion to further promote the oxidation reaction of the unburned components of the exhaust gas. Further, the material of the catalyst may be different in each part.

In addition, even if only the specific tip portion is made dense, the same effect as that of the present invention can be obtained. Examples of the means for making the particles dense include strengthening the thermal spraying condition only at the specific tip portion, and making the thermal spraying powder finer. However, strictness is required for manufacturing control as compared with the specification of the thickness according to the present invention.

Examples of the method for forming the porous protective layer include various methods such as brushing a solution or powder of the material, followed by firing such as brush application, dipping, spraying, etc., but plasma spraying is particularly preferable. In the case of plasma spraying, it is advisable to spray the specific tip portion and the rear portion together and then further spray only the specific tip portion, or the specific tip portion and the rear portion separately.

(Example) Hereinafter, the Example of this invention is described.

In FIGS. 1 and 2, 1 is an oxygen sensor element,
This element 1 comprises a solid electrolyte 2 capable of causing a difference in oxygen concentration between a reference gas and exhaust gas, and a pair of porous electrodes (inner electrodes) 3 and (outer electrodes) 4 formed on the inner and outer surfaces of the solid electrolyte.
And a porous protective layer 5 covering the outer electrode 4. Here, the solid electrolyte 2 is composed of ZrO ₂ added with Y ₂ O ₃ , both electrodes 3 and 4 are Pt electrodes, the protective layer 5 is formed of spinel, and Pt particles 5a ... There is. In addition, 6 is a metal case as a holding body of the element 1.

The outer electrode (or its lead) has the axial length (L).
From the element detecting portion 1a corresponding to the above, to the output extracting portion through the element expanded portion 1b as a mounting portion to the metal case 6.
The protective layer 5 covers the entire area of the detecting portion 1a (axial direction and circumferential direction), that is, covers the outer electrode so that the rear end of the protective layer 5 reaches the transition portion from the detecting portion 1a to the expanded diameter portion 1b. There is. It should be noted that the rear end of the detection unit 1a and the rear end of the protective layer 5 do not necessarily have to coincide with each other, but it is necessary to provide the protective layer 5 at least in the portion where the electrode 4 is directly exposed to the exhaust gas. is there.

In Fig. 1, the length along the axial direction of the solid electrolyte element of the porous protective layer formed from the element tip to the transition portion to the expanded diameter portion 1b is indicated by L (here, the axial length of the detection portion 1a and Same). The protective layer 5 has a portion at a certain distance in the axial direction from the tip, that is, the thickness of the specific tip portion 5a is larger than that of the rear portion 5b. In FIG. 1, the axial length of the specific tip 5a is indicated by l. Therefore, in the present invention, L and l have the following relationship as described above.

1 / 5L ≤ l ≤ 1 / 2L Here, the one with l ≒ 2 / 5L is shown.
5a and the rear portion 5b are each made to have a substantially uniform thickness (however, the boundary portion thereof has a tapered shape in which the front side is made thicker).

Next, a production example of the oxygen sensor element of the present invention will be described.

Process 1. For ZrO ₂ raw material with a purity of 99% or higher, a purity of 99.9% corresponding to 5 mol%
Add Y ₂ O ₃ .

Step 2. Add water and wet-mix in a ball mill for 70 hours and crush.

Step 3. After drying, heat-treat at 1300 ℃ × 2Hr.

Step 4. After heat treatment, pulverize 200Hrs by wet process, add 10Hrs of binder mixture, and spray dry to obtain particle size of 50 ~ 8
0 μm spherical granulated particles are obtained. -(Engineering) 4 Step 5. The powder obtained in (Engineering) 4 is rubber-pressed to form a desired tubular shape (test tubular shape), which is dried and then ground into a predetermined shape with a grindstone.

Step 6. Thereafter, on the outer surface, a slurry in which the water-soluble binder fibrin sodium glycolate and the solvent were added to the granulated particles obtained in (work) 4 was adhered, dried, and calcined at 1500 ° C. × 2 Hrs. The axial length of the part corresponding to the detection part 1a is
25mm, outer diameter about 5mmφ, inner diameter about 3mmφ.

Process 7. After firing, chemical plating is used to form a Pt layer with a thickness of 0.9μ on the outer surface.
m was deposited and then baked. Further, as a protective coat, MgO.Al ₂ O ₃ (spinel) powder was plasma-sprayed to form a porous protective layer having a thickness of about 100 μm.

Further, as an additional thermal spraying process, as shown in the table below, the length of the specific tip portion 5a is variously changed to plasma-spray spinel powder to increase the thickness of the specific tip portion 5a to about 100 μm and the total thickness of 200 μm.
It was set to m.

Process 8. After that, the same chemical plating as in Process 7 was performed on the inner surface, and then the baking treatment was performed.

Process 9. The element was incorporated into a stainless steel fitting and made into a sensor shape.

The adhesion of the granulated particles in the step 6 above forms spherical projections on the surface of the solid electrolyte to enhance the binding force between the solid electrolyte and the outer electrode. For details, see Japanese Patent Application No. 62-20.
See 0071. After step 8, a catalyst supporting step, for example, dipping in a H ₂ PtCl ₆ aqueous solution and heat treatment at 600 ° C. may be performed.

The oxygen sensor element of the present invention according to the above production example was examined for each of the following evaluation items. As a comparative example, the thickness of the entire detecting portion 1a is 100 μm (without additional irradiation: sample No. 8),
200 μm (additional sprayed over the entire area: sample No. 9) is also shown.

Evaluation item A (Chemical noise): Sensor output voltage width [mV] at idling (800 rpm); Evaluation item B (λ point deviation): NO _X emission amount at 80 km / H [g / mile] Evaluation item C (response): frequency [Hz] during sensor control (note that response (response time) = (1000 / frequency) msec).

Referring to the above table, sample No. 1 (comparative example) having a short specific tip portion (l / L = 1/6), which is considered to be thick, and a uniform thickness without specific tip portion (l / L = 0) ) Sample No. 8 (Comparative Example) is greatly affected by the chemical size, and therefore the output voltage width in Evaluation A is extremely narrow.

On the other hand, a sample with a very long specific tip (l / L> 1/2)
In Nos. 6, 7 and 9 (all comparative examples), the output voltage range was secured in evaluation A, but in the evaluation B, the amount of NOx in the exhaust gas was 0.28 g / mile or more, and the examples (No. 2-5)
Compared with the above, the discharge flow has increased remarkably.

Further, in the evaluation C, the responsiveness of each of the examples (Nos. 2 to 5) exceeds 1.9 Hz, which is a value that satisfies the requirement for control responsiveness. As is clear from the above table, the samples (Nos. 2 to 5) according to the examples of the present invention show excellent results for all of the evaluation items A, B, and C.

Although the step of providing the specific tip portion is performed as an additional step in the step of this example, it is industrially advantageous that the porous protective layer is integrally formed at the same time.

(Effect) As described above, according to the present invention, since the thickness of the porous body itself is made large at the specific tip portion of the porous protective layer, the atmosphere is averaged (mixed uniformly in the protective layer) to prevent the chemical noise phenomenon. This can be prevented, and the accuracy of the air twist ratio control can be improved.

In addition, since the wall thickness of only the specific tip is good, the responsiveness can be maintained even at high temperature, and the NO _X emission amount can be maintained at a low level (no λ point deviation occurs). The accuracy is improved.

Therefore, the oxygen sensor element of the present invention is particularly effective for suppressing the chemical noise phenomenon at low temperature and for restricting the increase of NO _X emission amount by lean control at high temperature, and is combined with the three-way catalyst. For example, it can contribute to a significant improvement in purification characteristics and is therefore extremely useful as an oxygen sensor for controlling the air-twist ratio.

Further, if a catalyst is supported on the protective layer, the above-mentioned effects can be maintained more stably, and the unburned components in the atmosphere can be balanced. Therefore, it can be used for a long period of time and the usability can be improved.

[Brief description of drawings]

FIG. 1 is a part showing an embodiment of the present invention (porous protective layer).
Sectional views and FIG. 2 are partially enlarged sectional views of FIG. 1, respectively. 1 ... Oxygen sensor element, 1b ... Mounting part 2 ... Solid electrolyte, 3 ... Reference electrode 4 ... Measurement electrode, 5 ... Porous protective layer 5a ... Specific tip part

Claims (7)

[Claims] 1. A tubular solid electrolyte having a closed front end and an open rear end provided with a reference electrode on one surface and a measurement electrode on the other end, and the measurement electrode is covered with a porous protective layer. An oxygen sensor element, wherein the front and rear ends of the porous protective layer are respectively located on the front end side and the rear end side of the solid electrolyte, and further, the tubular solid electrolyte axial direction from the front end of the porous protective layer. A portion up to a distance 1 along the above is defined as a specific tip portion of the porous protective layer, and the total length along the axial direction from the tip to the rear end of the porous protective layer is defined as L / 5 ≦ l ≦ There is a relationship represented by L / 2, the thickness of the specific tip portion, by increasing the thickness of the porous protective layer itself, to the porous protective layer thickness other than the specific tip portion. , At least 1.5 times the average thickness of the oxygen sensor element 2. The thickness of the specific tip portion is at least 1.5 to 2 times the average thickness as compared with the thickness of the porous protective layer other than the specific tip portion. 2. The oxygen sensor element according to item 1 above. 3. The average thickness of the specific tip portion is formed to be 80 μm or more thicker than the average thickness of the porous protective layer other than the specific tip portion.
The oxygen sensor element according to the item. 4. The oxygen sensor element according to claim 1, wherein the porous protective layer does not include a catalyst layer. 5. The oxygen sensor element according to claim 1, wherein the specific tip portion is formed more densely than the porous protective layer other than the specific tip portion. 6. The thickness of the porous protective layer is gradually increased toward the specific tip portion, and the average thickness of the specific tip portion is equal to that of the porous protective layer other than the specific tip portion. The oxygen sensor element according to claim 1, wherein the oxygen sensor element is formed to be 50 μm or more thicker than the average thickness. 7. The relationship expressed by L '/ 2≤l≤9L' / 10, where L'is the total length along the axial direction of the portion of the oxygen sensor element that directly contacts the gas. The oxygen sensor element according to claim 1, wherein: