JP2003315304A - Oxygen sensor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly reliable oxygen sensor element capable of preventing the occurrences of a crack due to a rapid temperature rise while maintaining the strength of the element. <P>SOLUTION: The oxygen sensor element comprises a sensor section having a plate-shaped solid electrolyte substrate 22 whose end is sealed in which an air inlet hole 22a is formed. A reference electrode 23 is formed on an inner wall of the air inlet hole 22a. A measuring electrode 24 of platinum is formed on a surface of the substrate 22 opposite to the reference electrode 23. The length Ws of an end face of the air inlet hole 22a located on the side of the reference electrode 23 as taken in a direction perpendicular to the direction of the length of the element inside the air inlet hole 22a is greater than the width W of the reference electrode 23 as taken in the direction perpendicular to the direction of the length of the element, and greater than the length Wh of the opposite end face of the air inlet hole 22a. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor element for controlling the ratio of air to fuel in an internal combustion engine such as an automobile.

[0002]

2. Description of the Related Art Currently, in an internal combustion engine of an automobile or the like, the oxygen concentration in the exhaust gas is detected, and the amount of air and fuel supplied to the internal combustion engine is controlled based on the detected value so that Harmful substances such as CO, H
A method of reducing C and NOx is adopted.

As this detecting element, a pair of electrode layers (outer surface: measuring electrode, respectively) are formed on the outer surface and the inner surface of a cylindrical tube mainly composed of zirconia having oxygen ion conductivity and having one end sealed. (Inner surface: reference electrode)
A solid electrolyte type oxygen sensor in which is formed is used.

In such an oxygen sensor, generally,
As a so-called theoretical air-fuel ratio sensor (λ sensor) used for controlling the ratio of air to fuel near 1, a ceramic porous layer is provided as a protective layer on the surface of the measuring electrode, and at a predetermined temperature. The air-fuel ratio of the engine intake system is controlled by detecting the oxygen concentration difference generated on both sides of the cylindrical pipe. At this time, the stoichiometric air-fuel ratio sensor needs to be heated to an operating temperature of about 700 ° C. Therefore, a rod-shaped heater is inserted inside the cylindrical tube to heat the sensor unit to the operating temperature.

[0005]

However, in recent years, the tendency to tighten exhaust gas regulations has become stronger, and it has become necessary to detect CO, HC, and NOx immediately after the engine is started. In response to such a demand, as described above, in the indirect heating type cylindrical oxygen sensor in which the heater is inserted into the cylindrical tube,
There is a problem that exhaust gas regulations cannot be sufficiently met because the time required for the sensor unit to reach the activation temperature (activation time) is slow.

In recent years, as a method for avoiding this problem, as shown in the schematic sectional view of FIG.
1. An oxygen sensor element in which a sensor portion provided with a measurement electrode 52 and a quasi electrode 53 on the outer surface of 1 and an inner surface of an air introduction hole 54 and a heater portion having a platinum heater 56 embedded in a ceramic insulating layer 55 are integrated. Is proposed.

However, unlike the above-described conventional indirect heating method, this heater-integrated oxygen sensor is capable of rapid temperature rise because it is a direct heating method, but it is originally formed in the air introduction hole 54. The reference electrode 53 and the lead pattern of the reference electrode 53 should be slightly misaligned at the time of stacking so that a part of the end portion of the reference electrode 53 and the lead pattern of the reference electrode 53 is embedded in the solid electrolyte substrate.
As a result, cracks and the like occur from the embedded part due to repeated thermal shock due to rapid temperature rise, shortening the life of the sensor element, resulting in a defective product.
It has been a major factor in lowering the production yield.

Further, in the oxygen sensor element, the air introduction hole 54 is an indispensable element, but there is a problem that the strength of the element itself is lowered by the air introduction hole 54.

Therefore, the present invention provides a highly reliable oxygen sensor element capable of preventing the occurrence of cracks due to rapid temperature rise while maintaining the strength of the element.

[0010]

As a result of studying the above problems, the present inventor has found that controlling the air introduction hole to have a predetermined shape can enhance the strength of the element and also endure it against a rapid temperature rise. The inventors have found that it is also possible to enhance the properties and have reached the present invention.

That is, the oxygen sensor element of the present invention is a flat plate body having an air introduction hole in which the tip of the solid electrolyte substrate is sealed, and a reference electrode is formed on the inner wall of the air introduction hole. In an oxygen sensor element having a sensor section in which a measurement electrode is provided on the surface of a solid electrolyte substrate facing the electrode, a bottom surface of the reference electrode formation side in a direction orthogonal to the longitudinal direction of the element inside the air introduction hole. The length is longer than the reference electrode width in a direction orthogonal to the longitudinal direction of the element, and is longer than the opposing bottom surface length in the air introduction hole, the oxygen sensor element, It is desirable to provide a heater section in which a heating element is embedded in the ceramic insulating layer.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION An example of the basic structure of an oxygen sensor element of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic cross-sectional view for explaining an example of the oxygen sensor element of the present invention, and FIG. 2 is a schematic cross-sectional view for explaining another example.
These are generally called theoretical air-twisting ratio sensor elements, and each of them includes a sensor section 20 and a heater section 21 in the examples of FIGS.

In the oxygen sensor element of FIG. 1, a solid electrolyte substrate 22 made of zirconia and having oxygen ion conductivity, and on both surfaces of the solid electrolyte substrate 22 facing each other,
A reference electrode 23 in contact with air and a measurement electrode 24 in contact with exhaust gas are formed to form a sensor unit 20 having a function of detecting oxygen concentration.

That is, the solid electrolyte substrate 22 has a flat plate-like hollow shape with a sealed tip, and this hollow portion forms an air introduction hole 22a. Then, this air introduction hole 22
A reference electrode 23 that contacts a reference gas such as air is deposited on the inner wall of a, and a measurement electrode that contacts a measurement gas such as exhaust gas is formed on the outer surface of the solid electrolyte substrate 22 facing the reference electrode 23. 24 are formed.

From the viewpoint of preventing poisoning of the electrode by exhaust gas, a ceramic porous layer 25 is formed on the surface of the measuring electrode 24 as an electrode protective layer.

According to the present invention, in such an oxygen sensor element, the bottom surface length Ws on the side where the reference electrode 23 is formed in the direction orthogonal to the longitudinal direction of the element inside the air introduction hole 22a.
However, it is important that the width is longer than the width W of the reference electrode 23 in the direction orthogonal to the longitudinal direction of the element and longer than the length Wh of the bottom surface facing each other. Bottom length Ws on the reference electrode 23 side
Becomes shorter than the width W of the reference electrode 23,
Both ends are embedded in the solid electrolyte substrate 22, and cracks are likely to occur due to thermal shock during rapid temperature rise. Further, if the bottom surface length Ws on the reference electrode 23 side and the width W of the reference electrode 23 are the same, the reference electrode 23 is embedded in the solid electrolyte substrate 22 due to a slight misalignment during stacking, so that the temperature rises rapidly. The thermal shock of causes cracks. On the other hand, if the bottom surface length Ws on the side of the reference electrode 23 is equal to or shorter than the length Wh of the opposed bottom surface, the strength of the element is reduced and the element may be broken in the engine, resulting in lack of reliability.

More specifically, the width W of the reference electrode 23, the bottom surface length Ws on the reference electrode 23 side, and the length Wh of the opposed bottom surface.
Means that W / Ws is 0.7 to 0.95 and Wh / Ws is 0.
A value of 5 to 0.95 is suitable.

In the present invention, both the reference electrode 23 and the measurement electrode 24 are made of porous platinum, and the zirconia phase exists in the platinum particles and the zirconia phase is 0.1 to 0.1 in the platinum particles. The content of 10% by volume can suppress the grain growth of platinum particles, and as a result, the measurement electrode 24 having good gas responsiveness can be formed.

On the other hand, the heater section 21 has a structure in which a heating element 27 such as platinum is embedded in a ceramic insulating layer 26 having an electric insulation property. In the oxygen sensor element of FIG. 1, the heater section 21 is a sensor section. The oxygen sensor element of FIG. 2 has a structure in which the sensor unit 20 and the heater unit 21 are formed separately from each other and joined by a joining material 28.

In particular, the solid electrolyte substrate 22 of the sensor section 20
When the difference in thermal expansion coefficient between the ceramic insulating layer 26 of the heater 21 and the ceramic insulating layer 26 is large, the structure shown in FIG. 2 is preferably used.
4 is not formed, it is desirable to join at a low temperature portion during use.

Further, when the sensor portion and the heater portion are bonded to each other over the entire surface, stress due to the difference in thermal expansion coefficient between the sensor portion 20 and the heater portion 21 is relaxed, so that, for example, the zirconia solid electrolyte of the sensor portion 20 is used. Substrate 22 and heater section 21
It is desirable to interpose a composite material of the alumina ceramic insulating layer 26, and a composite compound layer of alumina and zirconia.

The solid electrolyte used in the oxygen sensor element of the present invention is preferably made of a ceramic containing ZrO ₂ , and Y ₂ O ₃ and Y ₂ are used as stabilizers.
_{b 2 O 3, Sc 2 O} 3, Sm 2 O 3, Nd 2 O 3, Dy 2 O 3 or the like 1 to 30 mol% of rare earth oxide in terms of oxide of the portion preferably containing 3 to 15 mol% Stabilized ZrO ₂ or stabilized ZrO ₂ is used.

Further, Zr in ZrO ₂ is 1 to 20 atomic%.
By using ZrO _{2 in} which is substituted with Ce, there is an effect that ionic conductivity is increased and responsiveness is further improved.

Further, for the purpose of improving the sinterability, the above Z
against rO _2, Al ₂ O ₃ and has a SiO ₂ may be contained additives, the inclusion in a large amount, since the creep characteristic is deteriorated at high temperatures, Al ₂ O ₃ and SiO ₂
It is desirable that the total amount of added is 5% by weight or less, especially 2% by weight or less.

The reference electrode 23 and the measurement electrode 24 deposited on the surface of the solid electrolyte substrate 22 are both platinum or an alloy of platinum and one alloy selected from the group of rhodium, palladium, ruthenium and gold. Used. Also,
At the time of sensor operation, the purpose of preventing the particle growth of the metal in the electrode, the platinum particles related to the response, the solid electrolyte and the gas,
For the purpose of increasing the number of contacts at the so-called three-phase interface, the above-mentioned ceramic solid electrolyte component is contained in an amount of 1 to 50% by volume, particularly 10 to 3%.
You may mix in the said electrode in the ratio of 0 volume%.

The shape of the electrodes may be square or elliptical. Further, the thickness of the electrode is preferably 3 to 20 μm, and particularly preferably 5 to 10 μm.

On the other hand, as the ceramic insulating layer 26 in which the platinum heating element 27 is embedded, it is desirable that the ceramic insulating layer 26 be made of dense ceramics having a relative density of 80% or more and an open porosity of 5% or less. .

At this time, for the purpose of improving the sinterability, the ceramic insulating phase 26 may contain Mg, Ca, and Si in a total amount of 1 to 10% by mass, but contains an alkali metal such as Na or K. As for the amount, the heater portion 2 is used due to migration.
50 pp in terms of oxide in order to worsen the electrical insulation property of 1.
It is desirable to control to m or less.

By setting the relative density within the above range, the strength of the substrate increases, and as a result, the mechanical strength of the oxygen sensor itself can be increased.

The ceramic porous layer 25 formed on the surface of the measuring electrode 24 has a thickness of 10 to 800 μm and is selected from the group consisting of zirconia, alumina, γ-alumina and spinel having a porosity of 10 to 50%. It is desirable to be formed of at least one kind.

When the thickness of the porous layer 25 is less than 10 μm or the porosity exceeds 50%, the electrode poisoning substances P, Si, etc. easily reach the electrode and the electrode performance is deteriorated. On the other hand, when the thickness of the porous layer 25 exceeds 800 μm or the porosity becomes smaller than 10%, the gas diffusion rate in the porous layer 25 becomes slow, and the gas responsiveness of the electrode deteriorates. In particular, the thickness of the porous layer 25 is preferably 100 to 500 μm, though it depends on the porosity.

Ceramic insulating layer 2 in heater portion 21
The platinum heating element 27 embedded in 6 can use platinum alone as a metal, or an alloy of platinum and one kind selected from the group of rhodium, palladium and ruthenium.

The heating pattern of the heating element 27 in the heater section 21 may be not only a structure extending in the longitudinal direction and being folded back at the end in the longitudinal direction, but also a meander structure.

In the oxygen sensor element of the present invention, the total thickness of the element is 0.8 to 1.5 mm, particularly 1.0 to
1.2 mm and the length of the element is preferably 45 to 55 mm, and particularly preferably 45 to 50 mm in view of the relationship between the rapid temperature rising property and the mounting condition of the element in the engine.

Next, a method of manufacturing the oxygen sensor element of the present invention will be described with reference to the exploded perspective view of FIG. 3 by taking the method of manufacturing the oxygen sensor element of FIG. 1 as an example.

First, a solid electrolyte green sheet 41 is prepared. This green sheet 41 is obtained by, for example, appropriately adding a molding organic binder to a ceramic solid electrolyte powder having oxygen ion conductivity of zirconia, a doctor blade method, an extrusion molding, or a hydrostatic molding (rubber press). Alternatively, it is produced by a known method such as press forming.

Next, on both surfaces of the green sheet 41, the patterns 42a, the lead patterns 42b, the pads 43a, which serve as the measurement electrode 24 and the reference electrode 23, are formed.
The through holes 43b and the like are formed by, for example, using a conductive paste containing platinum by a slurry dip method, screen printing, pad printing, or roll transfer.

Next, a green sheet 45 and a green sheet 46 having the air introduction hole 44 formed therein are prepared. According to the present invention, the air introduction hole 44 is processed into a cross-sectional shape as shown by the air introduction hole 22a in FIG. For example,
As shown in the cross-sectional view of FIG. 4A, after the air introduction hole 44 is formed in the green sheet 45 in advance by punching or the like, as shown in FIG. By pressing the mold b, a taper a having a predetermined angle is formed.

Thereafter, the green sheet 45 and the green sheet 46 having the air introduction holes 44 formed therein are made to interpose an adhesive such as an acrylic resin or an organic solvent to the green sheet 41, or while applying pressure with a roller or the like. A laminate A for the sensor unit is produced by mechanically adhering.

Further, as the platinum-containing conductive paste used at this time, 1 to 50% by volume, especially 10 to 10% by volume of zirconia composed of the above-mentioned ceramic solid electrolyte component is used.
It is desirable to use platinum particles that are included at a ratio of 30% by volume and to additionally contain organic resin components such as ethyl cellulose. By using such platinum particles containing ZrO2, it is possible to suppress the sintering of the platinum particles when the sensor is used at a high temperature.
It is possible to maintain a stable sensing function.

Next, as shown in FIG. 3, a paste made of alumina powder is printed on the surface of the zirconia green sheet 47 by a slurry dip method, screen printing, pad printing or roll transfer to form a ceramic insulating layer 48a.

Next, on the surface of the ceramic insulating layer 48a,
The heating pattern 49 and the lead pattern 50 are applied by printing. Then, an insulating paste such as alumina is applied and the ceramic insulating layer 48b is applied by printing,
The laminated body B of the heater portion 21 is produced.

In producing the laminated body of the heater portion 21, the ceramic insulating layers 48a and 48b are formed by printing and applying the insulating paste as described above, or by using a ceramic slurry such as alumina. An insulating sheet can be formed and laminated by a sheet forming method such as a doctor blade method.

Thereafter, the laminated body A of the sensor portion 20 and the laminated body B of the heater portion 21 are made to interpose an adhesive such as an acrylic resin or an organic solvent, or they are mechanically adhered while applying pressure with a roller or the like. These are fired after they are bonded and integrated. The firing is performed in the temperature range of 1300 ° C. to 1700 ° C.
Bake for 10 hours. In addition, in order to suppress the warpage of the sensor unit A during firing, the amount of warpage can be reduced by placing a smooth substrate such as alumina as a weight on the laminate.

Further, when the laminated body A of the sensor portion and the laminated body B of the heater portion are simultaneously fired and integrated, in order to reduce the generation of stress due to the difference in thermal expansion coefficient between the two, for example, a sensor is used. It is desirable to interpose a composite material of a solid electrolyte component forming the part and an insulating component forming the ceramic insulating layer of the heater part.

Thereafter, if necessary, at least one ceramic selected from the group consisting of alumina, zirconia, and spinel is formed on the surface of the measurement electrode after firing by plasma spraying or the like to integrate the heater portion. Oxygen sensor elements can be formed.

In the above method, the heater portion 21 was formed by firing the sensor portion 20 at the same time. However, after the sensor portion 20 and the heater portion 21 are fired separately, they are made of glass or the like. It is also possible to integrate them by joining with a suitable inorganic joining material.

[0048]

EXAMPLE The stoichiometric air-fuel ratio sensor shown in FIG. 1 was manufactured as follows according to FIG.

First, commercially available alumina powder having a purity of 99.9%, zirconia powder containing 5% by weight of Y ₂ O ₃ containing 0.1% by weight of Si, and 8% by mole of an average particle diameter of 0.1 μm. A platinum powder containing 30% by volume of zirconia composed of yttria and a platinum powder containing 20% by volume of alumina powder were prepared.

First, a polyvinyl alcohol solution was added to zirconia powder containing 5 mol% Y ₂ O ₃ to prepare a slurry, and a zirconia green sheet having a thickness after sintering of 0.4 mm was obtained by extrusion molding. 41 was produced.

After that, a conductive paste containing platinum powder is screen-printed on both sides of the green sheet 41 to form the measurement electrode and reference electrode patterns 42a, lead patterns 42b, pads 43a, and through holes 43b, respectively. The green sheet 45 having the air introduction hole 44 formed therein and the green sheet 46 were laminated with an acrylic resin adhesive to obtain a sensor unit laminate A. In addition, as shown in FIG. 4, a taper a was appropriately formed on the edge portion of the air introduction hole 44 by using a predetermined mold.

Next, the surface of the zirconia green sheet 47 is screen-printed with the paste made of the above-mentioned alumina powder to burn the ceramic insulating layer 48a, and then the ceramic insulating layer 48a is burned to about 10 μm.
m, the heating pattern 49 and the lead portion 50 are formed by screen printing using a conductive paste containing platinum containing alumina, and a paste made of alumina powder is again formed on this surface. The ceramic insulating layer 48b was screen-printed to form the heater unit laminate B.

Thereafter, the laminated body A for the sensor portion and the laminated body B for the heater portion are joined according to the above-described manufacturing method, and the laminated body of the heater integrated sensor element is fired at 1500 ° C. for 1 hour to form the heater integrated sensor element. Was produced.

At this time, by changing the widths of the reference electrode and the upper and lower bottom surfaces of the air introduction hole, the theoretical air-fuel ratio type (λ
(Type) heater integrated oxygen sensor element was produced.

For each of the 50 oxygen sensor elements produced under each of these operating conditions, the positional deviation between the reference electrode and the air introduction hole was confirmed by ultrasonic flaw detection, and the stacking yield was determined. Further, with respect to 20 non-defective products, 4-point bending strength was measured and an average value was calculated.

After that, the air-fuel ratio was set to 12 and 2 by using a mixed gas of hydrogen, methane, nitrogen, and oxygen for 20 non-defective products in which the reference electrode and the air introduction hole were not misaligned by stacking.
By applying 12 V to the heater of the element for 100 hours while alternately spraying the mixed gas of 3 onto the sensor element at intervals of 0.5 seconds, it was confirmed whether or not the element was damaged, and the number of damaged elements is shown in Table 1. . The results are shown in Table 1.

[0057]

[Table 1]

From the results shown in Table 1, it can be seen from Sample No. 3 that the upper bottom length Ws of the air introduction hole of the device is equal to or shorter than the reference electrode width W. Nos. 1 to 6 had a large proportion of stacking faults, and all had a yield of 50% or less.

Even if the reference electrode width W is shorter than the length Ws of the bottom surface of the air introduction hole, Ws is the same as Wh, or Ws <Wh. Both Nos. 11 and 12 had low 4-point bending strength, and were damaged by applying 12 V to the heater of the element to rapidly raise the temperature.

On the other hand, all the products of the present invention had a high yield and were not damaged even by the 12V application test.

[0061]

As described in detail above, according to the present invention, by controlling the air introduction hole to a predetermined size, it is possible to prevent the occurrence of cracks due to rapid temperature rise while maintaining the strength of the element. It is possible to provide a highly reliable oxygen sensor element.

[Brief description of drawings]

FIG. 1 is a schematic sectional view for explaining an example of an oxygen sensor element of the present invention.

FIG. 2 is a schematic sectional view for explaining another example of the oxygen sensor element of the present invention.

FIG. 3 is an exploded perspective view for explaining a method of manufacturing the oxygen sensor element of FIG.

FIG. 4 is a diagram for explaining a method of forming an air introduction hole in the present invention.

FIG. 5 is a schematic cross-sectional view for explaining the structure of a conventional heater-integrated oxygen sensor element.

[Explanation of symbols]

1, 22 ... Solid electrolyte substrate 1a, 22a ... Atmosphere introduction hole 2, 24 ... Measuring electrodes 3, 23 ... Reference electrode 4, 25 ... Ceramic porous layer 5, 26 ... Ceramic insulating layer 6,27 ・ ・ ・ Platinum heating element 20 ... Sensor part 21 ・ ・ ・ Heater part 28 ... Bonding material

Claims (2)

[Claims] 1. A flat plate body having an air introduction hole in which a front end portion of a solid electrolyte substrate is sealed, wherein a reference electrode is formed on an inner wall of the air introduction hole, and the solid electrolyte substrate is opposed to the reference electrode. In an oxygen sensor element having a sensor portion provided with measurement electrodes on the surface, the bottom length of the reference electrode formation side in the direction orthogonal to the longitudinal direction of the element inside the air introduction hole is the longitudinal direction of the element. Is longer than the reference electrode width in a direction perpendicular to the above, and longer than the opposing bottom surface length in the air introduction hole. 2. The oxygen sensor element according to claim 1, further comprising a heater section in which a heating element is embedded in a ceramic insulating layer.