JP2002257771A - Oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the responsiveness of an oxygen sensor equipped with a measuring electrode formed by applying a paste containing platinum group metal followed by baking. SOLUTION: This oxygen sensor 1 comprises the measuring electrode 4 and a reference electrode 3 provided on the opposed surfaces of a zirconia solid electrolytic base 2. The measuring electrode 4 contains platinum group metal and zirconia, and the area resistance is set to 0.3-6 Ω/mm<2> , or the total length with thickness of 6 μm or less is set to 50% or more of the whole length with the longest diameter when the section laid along the longest diameter in the plane of the measuring electrode 4 is observed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an oxygen sensor, and more particularly to an oxygen sensor for controlling the ratio of air to fuel in an internal combustion engine of an automobile or the like.

[0002]

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

As shown in FIG. 6, for example, as shown in FIG. 6, a reference portion made of platinum as a sensor portion and in contact with a reference gas such as air is provided on the inner surface of a cylindrical tube 41 having a sealed tip made of a zirconia solid electrolyte. An electrode 42 is formed on the outer surface of the cylindrical tube 41, and a measurement electrode 43 that is in contact with a gas to be measured, such as exhaust gas, is formed on the surface of the measurement electrode 43. A sensor is used. Such an oxygen sensor is generally used as a so-called stoichiometric air-fuel ratio sensor (λ sensor) used for controlling the ratio of air and fuel to around 1, and the oxygen concentration generated on both sides of the cylindrical tube 41 at a predetermined temperature. The difference is detected. The ceramic porous layer 44
Function as a gas diffusion-controlling layer, apply an applied voltage to a cylindrical tube 41 made of a solid electrolyte through a pair of electrodes 42 and 43, measure a limit current value obtained at that time, and control an air-fuel ratio in a lean burn region. It is used as an air-fuel ratio sensor (A / F sensor).

[0004] Both the stoichiometric air-fuel ratio sensor and the wide-range air-fuel ratio sensor need to heat the detecting portion to an operating temperature of about 700 ° C.
A rod-shaped heater 45 is inserted to heat the detector to the operating temperature.

On the other hand, in recent years, the exhaust gas regulations have been strengthened, and it has become necessary to detect CO, HC, and NOx immediately after the start of the engine, and the time required for the detection unit to reach the activation temperature (hereinafter referred to as activation). ) Is required.

However, in the indirect heating type cylindrical oxygen sensor in which the heater 45 is inserted into the cylindrical tube 41 as described above, it is difficult to detect CO or the like immediately after the engine is started because the activation time is long. However, there is a problem that it is not possible to sufficiently cope with exhaust gas regulations. To this end, an oxygen sensor is disclosed in Japanese Patent Application Laid-Open No. H10-27083, in which a sensor section having a reference electrode and a measurement electrode provided on the inner and outer surfaces of a solid electrolyte cylindrical tube and a heater section having a platinum heating resistor embedded in an insulator are integrated. 10-
206380 and JP-A-13-13101.

[0007]

Although such a heater-integrated oxygen sensor differs from the conventional indirect heating system in that it employs a direct heating system, it is possible to rapidly raise the temperature. In the case of manufacturing by simultaneously firing the heater portion and the electrode, the electrode is formed by applying a paste containing platinum powder to the surface of the solid electrolyte molded body and firing the electrode, and is formed using such a platinum paste. Since the electrode is sintered at a high temperature, the platinum particles sinter together and grow, and the interface between the platinum particles, the solid electrolyte, and the gas phase, the number of so-called triple points, is reduced, and the gas to be measured is reduced. However, there is a problem that gas responsiveness is deteriorated.

Accordingly, an object of the present invention is to improve the gas responsiveness of an oxygen sensor having a measurement electrode formed by applying and firing a paste containing a platinum group metal.

[0009]

Means for Solving the Problems The present inventors formed a measurement electrode from a composite material of a platinum group metal and zirconia, and controlled the gas responsiveness by controlling the sheet resistance to 0.3 to 6 Ω / mm ² . Can be improved. In particular, in order to set the sheet resistance within the above range, the total length of the thickness of 6 μm or less when observing a cross section along the longest diameter in the plane of the measurement electrode is 50% or more of the total length of the longest diameter. Is desirable.

According to the present invention, such a measuring electrode is formed by applying a paste of a cermet powder containing a platinum group metal and zirconia to the surface of a solid electrolyte,
It is desirable to form by firing.

[0011] By integrally forming the ceramic insulating layer in which the heating resistor is embedded, the temperature of the oxygen sensor can be rapidly increased, and the performance of the oxygen sensor can be further improved.

Usually, in a measuring electrode formed by applying and baking a paste containing a platinum group metal powder, a gas to be measured coming into contact with the measuring electrode passes through the platinum group metal particles constituting the measuring electrode. Although it will penetrate from the surface of the measurement electrode to the inside, if the thickness of the measurement electrode made using a paste containing a platinum group metal is thick, a chemical reaction between the component gases occurs first at the upper part of the measurement electrode, so that platinum The gas supplied to the interface between the group metal particles, the solid electrolyte substrate, and the gas phase, ie, the triple point at which a chemical reaction should originally occur,
The composition of the gas to be measured has already changed, and as a result, the oxygen sensor does not show a predetermined electromotive force, or the response to the gas is observed with an apparent delay.

According to the present invention, by controlling the sheet resistance of the measuring electrode in the range of 0.3 to 6 Ω / mm ² ,
The above gas responsiveness can be improved. Specifically, by controlling the thickness of the measurement electrode, it is possible to shorten the path until the gas to be measured diffuses to the triple point of the interface between the platinum group metal particles, the solid electrolyte substrate, and the gas phase, and Since the platinum group metal particles are formed by fine particles without grain growth, and have excellent gas diffusivity in the measurement electrode, a large amount of the gas to be measured which is hardly changed is supplied to the triple point. Gas response can be improved.

[0014]

FIG. 1 is a schematic perspective view showing a cylindrical oxygen sensor as an example of the oxygen sensor of the present invention, and FIG.
1 shows a cross-sectional view of the oxygen sensor shown in FIG. However, in FIG. 1, the ceramic protective layer 13 is omitted for convenience of explanation.

The oxygen sensor 1 shown in FIGS. 1 and 2 has a zirconia solid electrolyte substrate, an inner surface of a cylindrical tube 2 having a sealed tip made of a zirconia solid electrolyte, a first electrode and a reference gas such as air. A reference electrode 3 to be contacted is formed and a measurement electrode 4 that is in contact with a gas to be measured such as exhaust gas is deposited as a second electrode at a position facing the reference electrode 3 with the cylindrical tube 2 interposed therebetween. Is formed. And
A detection unit is formed by the reference electrode 3, the cylindrical tube 2 made of a zirconia solid electrolyte, and the measurement electrode 4.

A ceramic insulating layer 6 of Al ₂ O ₃ or the like is formed on the outer surface of the cylindrical tube 2, and a part or all of the measuring electrode 4 is exposed on the ceramic insulating layer 6. Opening 7 is formed as described above. Further, a heating resistor 8 made of Pt or the like for heating the detecting portion is buried around the opening 7 of the ceramic insulating layer 6.

On the surface of the ceramic insulating layer 6, Zr is added to further increase the heating efficiency of the heating resistor 8.
A ceramic heat insulating layer 9 made of O ₂ , Al ₂ O _3, or the like is laminated.

Reference electrode 3 formed on the inner surface of cylindrical tube 2
Is a sensor terminal portion 11a provided on the outer surface of the cylindrical tube 2 via the inner surface of the cylindrical tube 2 and the end surface of the open end of the cylindrical tube 2.
It is connected to the. On the other hand, the measuring electrode 4 formed on the outer surface of the cylindrical tube 2 is connected to the lead 10 formed on the surface of the ceramic heat insulating layer 9 via the end face of the opening 7, and further connected to the surface of the ceramic heat insulating layer 9. Terminal part 11 formed
b. In addition, the edge portion of each end face in the cylindrical tube 2 is chamfered so as to avoid electrical connection failure occurring at the edge portion.

A protective layer 12 made of ZrO ₂ or the like is further formed on the surface of the lead portion 10 formed on the surface of the ceramic heat insulating layer 9. The protective layer 12 can protect the lead portion 10 from physical destruction such as scratching when assembling the element or collision with foreign matter when the element falls. This protective layer 12 is preferably made of the same ZrO ₂ as the solid electrolyte in order to prevent the occurrence of stress due to a difference in thermal expansion with the solid electrolyte.

Further, as shown in FIG. 2, a porous ceramic protective layer 13 is formed so as to cover the entire outer surface of the detecting portion and the ceramic heat insulating layer 9.

Further, metal members 14 for connection to an external circuit are fixed to the sensor terminals 11a and 11b by, for example, Au-Cu brazing. As a result, the detection data generated in the detection section is transmitted to the external circuit via the lead section 10, the sensor terminal sections 11a and 11b, and the metal member 14.

On the other hand, the heating resistor 8 formed in the ceramic insulating layer 6 is formed so as to penetrate the lead portion 16 also formed in the ceramic insulating layer 6 and the ceramic insulating layer 6 and the ceramic heat insulating layer 9. The through conductors (not shown) are electrically connected to the heater terminal portions 18 formed on the outer surface of the ceramic heat insulating layer 9. A metal member 19 for connecting to an external heat-generating power supply is fixed on these heater terminal portions 18 with a brazing material, and an electric current is passed through the heat-generating resistor 8 through these, thereby heating the heat-generating resistor 8, The temperature of the detection unit including the measurement electrode 4, the cylindrical tube 2, and the reference electrode 3 is rapidly increased to a predetermined temperature.

The heating resistor 8 around the measuring electrode 4 is shown in FIG.
As shown in the figure, the openings are uniformly and symmetrically patterned on both sides of the opening 7. The heating resistor 8 is connected to the lead 1
6, the heater element 8 is heated by passing a current through the heater element 8 through the heater terminal section 18, and the heating element 8 is detected by the cylindrical tube 2, the reference electrode 3, and the measurement electrode 4. In this case, the lead portion 16 of the heating resistor 8 has a wide 1
Although it is possible to form it with two lines, by forming it with two or more lines, it is possible to enhance the bonding with the upper and lower ceramic insulating layers 6 sandwiching the lead portion 16 and increase the strength of the element. .

Further, as the overall size of the oxygen sensor, it is preferable that the oxygen sensor be formed of a cylindrical body having an outer diameter of 3 to 6 mm, particularly 3 to 4 mm, thereby reducing power consumption and improving sensing performance.

In the oxygen sensor of the present invention, the measuring electrode 4 in the detecting section contains a platinum group metal and zirconia. According to the present invention, the area resistance of the measuring electrode 4 is 0.3 to 6 Ω /. It is important to control to mm ² .
This is because, if the sheet resistance is smaller than 0.3 Ω / mm ² , the platinum group metal particles are sintered with each other, the porosity decreases, and the gas responsiveness deteriorates. Conversely, if the sheet resistance exceeds 6 Ω / mm ² , the electrical conductivity becomes poor, resulting in poor gas responsiveness. The area resistance of the measurement electrode is particularly excellent in the range of 0.8 to 5 Ω / mm ² from the viewpoint of gas responsiveness. In the present invention, the sheet resistance is defined as a value obtained by measuring the electric resistance between both ends of the longest diameter on the plane of the measurement electrode and dividing the resistance by the area of the electrode.

According to the present invention, when controlling the sheet resistance, the total length of the portion where the electrode thickness is 6 μm or less when observing a cross section along the longest diameter in the plane of the measurement electrode 4 is as follows: It is desirable that the length is 50% or more of the total length of the longest diameter. Specifically, as shown in FIG. 3, the longest diameter L ₀ of the measurement electrode 4 is a vertex X, when the measurement electrode 4 is rectangular,
This corresponds to a diagonal line connecting Y. The cross section along the diagonal line XY is observed, and the total length Lz (= L ₁ + L ₂ + L ₃ +...) Of the portions L ₁ , L ₂ , and L ₃ where the thickness of the measurement electrode 4 is 6 μm or less.
·) Divided by the longest diameter L ₀ .

This is because when the length ratio is less than 50%, that is, when the thickness of the measurement electrode is large, the gas to be measured is, as described above, the gas of the platinum group metal particles, the solid electrolyte substrate, and the gas phase. Before reaching the interface, that is, the triple point where a chemical reaction should originally occur, a chemical reaction of the gas to be measured occurs at the upper part of the measurement electrode, and the gas supplied to the triple point changes in the composition of the gas to be measured. Problems such as the oxygen sensor not exhibiting a predetermined electromotive force or an apparently delayed response to gas occur. In particular, from the viewpoint of gas responsiveness, it is desirable that the total length of the portion where the electrode thickness is 6 μm or less is 60% or more of the total length of the longest diameter.

As described above, in order to control the thickness and the sheet resistance of the measurement electrode, it is important that the measurement electrode is formed of platinum group metal particles and zirconia, and that zirconia is uniformly dispersed in the measurement electrode. . That is, by uniformly dispersing zirconia, sintering and grain growth between platinum group metals are prevented, and the sheet resistance can be controlled by forming appropriate voids. As described above, in order to uniformly disperse zirconia, there is a limit to the dispersion of zirconia powder in a normal mixed powder of a platinum group metal powder and zirconia powder.

Therefore, according to the present invention, it is desirable to use a cermet powder composed of a composite of a platinum group metal and zirconia. The cermet powder is prepared, for example, by mixing a mixture of a platinum group metal powder and a zirconia powder with 1600 to 17
After baking at 00 ° C., it is produced by grinding.

Further, according to the present invention, in order to form a thin measuring electrode, the paste is adjusted by increasing the solvent component in the conductive paste containing the platinum group metal and zirconia at the time of coating, as compared with the prior art. Then, it is applied by using a soft brush or the like, or its conductive paste is sprayed to be applied and formed on the surface of the solid electrolyte. At this time, it is desirable that both the metal powder and the zirconia powder contained in the paste are fine powders having an average particle diameter of 2 μm or less, particularly 1.5 μm or less.

In particular, it is most desirable to use a cermet powder having an average particle size of 0.1 to 2 μm. By applying a paste containing a fine powder composed of such a fine cermet of a platinum group metal and zirconia, and baking to form a measurement electrode, the zirconia uniformly dispersed in the sintering process becomes a platinum group metal. Sintering and grain growth can be prevented, and the thickness can be prevented from increasing with the grain growth. As a result, it is possible to obtain a measurement electrode having a small thickness made of fine platinum group metal particles and zirconia and having a predetermined porosity.

In the conductive paste, the composition ratio of the platinum group metal and zirconia is 1 to 50% by volume, preferably 10 to 50% by volume, based on the total of the platinum group metal and zirconia.
It is desirable to include the zirconia in an amount of 30% by volume.
If the volume ratio is less than 50% by volume, the sheet resistance is reduced.
If the number is larger than the above, the sheet resistance increases. Further, when preparing the paste, in order to control the thickness of the electrode to be thin, it is appropriate that the amount of the solvent in the paste is 50 to 100 parts by weight based on 100 parts by weight of the total amount of the metal and zirconia.

The platinum group metal in the present invention includes platinum, rhodium, ruthenium and osmium, and the measuring electrode is preferably made of platinum.

According to the present invention, the reference electrode 3 formed on the inner surface of the cylindrical tube 2 is also made of one or more alloys selected from platinum group metals, like the measurement electrode 4. Can be In particular, platinum is most desirable from the viewpoint of gas responsiveness.

Further, the reference electrode 3 only needs to be formed on the inner surface portion facing the portion exposed from the opening 7 of the measurement electrode 4, and has an area larger than the exposed portion area of the measurement electrode 4, for example, a cylinder. It may be formed on the entire inner surface of the tube 2.

(Solid Electrolyte) In the present invention, the cylindrical tube 2
The zirconia solid electrolyte used to form
It is made of ceramics containing rO ₂ , and specifically,
Y ₂ O _{3 ,} Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , D
1 to 30 mol% of rare earth oxide such as y ₂ O ₃ in terms of oxide, partially stabilized Zr which preferably contains 3 to 15 mol%
O ₂ or stabilized ZrO ₂ is used. Also, Z
Zr in which 1 to 20 atomic% of Zr in rO ₂ is substituted with Ce
Use of O ₂ has the effect of increasing the electron conductivity and further improving the responsiveness.

Further, in order to improve the sinterability, the above Z
Al ₂ O ₃ or SiO ₂ can be added to and contained in rO ₂ , but if it is contained in a large amount, the creep characteristics at high temperatures deteriorate, so that Al ₂ O ₃ and SiO ₂
Is preferably 5% by weight or less, particularly 3% by weight or less in total.

The content of Na in the solid electrolyte is 200 ppm or less, particularly 100 ppm, from the viewpoint of preventing the diffusion of the solid electrolyte into the ceramic insulating layer 6.
Is desirable.

(Ceramic Insulating Layer) As the ceramic insulating layer 6 in which the heating resistor 8 is embedded, an oxide containing alumina and / or magnesia, particularly, an alumina material, a spinel material, or a composite compound material of alumina and spinel is used. Is preferably used. At this time, in order to improve the sinterability of the ceramic insulating layer 6, it is desirable to add a small amount of a Si component. , S
If the content of i exceeds 5% by weight, the ceramic insulating layer 6
Since the diffusion and segregation of Na in the steel are promoted and the life of the heating resistor 8 made of platinum or the like is easily reduced, the Si content is set to 0.1.
A range of 1 to 5% by weight is desirable. As the Si content,
0.5 to 3% by weight is desirable. In particular, 0.5 to 2% by weight
Is desirable from the viewpoint of preventing the diffusion of Na.

The ceramic insulating layer 6 is desirably made of dense ceramic having a relative density of 80% or more and an open porosity of 5% or less. This is because the denseness of the ceramic insulating layer 6 increases the strength of the insulating layer 6, thereby increasing the mechanical strength of the oxygen sensor itself. Further, the content of Na in the ceramic insulating layer 6 is 50 ppm, particularly 3 ppm.
0 ppm or less is desirable for extending the life of the heater.

(Heating Resistor) The heating resistor 8 embedded in the ceramic insulating layer 6 is made of platinum,
1 selected from the group of rhodium, palladium and ruthenium
It is desirable to be made of a kind of metal or an alloy of two or more kinds. In particular, in view of co-sintering with the ceramic insulating layer 6,
It is desirable to select a metal or alloy having a melting point higher than the firing temperature of the ceramic insulating layer 6.

The heating resistor 8 contains, in addition to the above-mentioned metals, alumina, spinel, a compound of alumina / silica, forsterite, or the above-mentioned electrolyte from the viewpoint of preventing sintering and increasing the adhesion to the insulating layer 6. It is desirable to mix the obtained zirconia and the like in a volume ratio of 10 to 80%, particularly 30 to 50%.

The ceramic heat insulating layer 9 formed on the surface of the ceramic insulating layer 6 in which the heating resistor 8 is embedded is preferably made of zirconia ceramics. The ceramic heat insulating layer 9 made of zirconia can alleviate the stress caused by the difference in thermal expansion and the difference in firing shrinkage between the solid electrolyte and the ceramic insulating layer 6 so that the thermal stress can be minimized. At this time, the distance between the cylindrical tube 2 and the heating resistor 8 and the distance between the ceramic heat insulating layer 9 and the heating resistor 8 are each 2 μm.
It is desirable that this is the case.

(Opening) In the present invention, the shape of the measuring electrode 4 exposed in the opening 7 is not particularly limited, and the opening 7 is symmetrical in the cylindrical tube 2. The thermal shock resistance can be improved by providing them at two locations. The expansion of the opening 7 is set in the range of 30 to 90 degrees with respect to the center of the cross section of the cylindrical tube 2, thereby suppressing the generation of thermal stress around the opening and increasing the heating efficiency of the heating resistor 8. Can be increased.

(Porous Layer) In the oxygen sensor of the present invention, as shown in FIG. 2, a porous ceramic protective layer 13 is formed on the surface of the measurement electrode 4 exposed in the opening 7. However, this ceramic protective layer 13 is made of zirconia,
The open porosity formed from at least one selected from the group consisting of alumina, magnesia and spinel is 5 to 30%
Of a porous body.

In the stoichiometric air-fuel ratio sensor (λ sensor), the ceramic protective layer 13 is provided in order to prevent the measurement electrode 4 from being poisoned by the exhaust gas and lowering the output voltage. In a wide area air-fuel ratio sensor (A / F sensor), the sensor functions as a gas diffusion rate-controlling layer.

Next, a method of manufacturing the oxygen sensor according to the present invention will be described with reference to FIG. (1) First, an organic binder for molding is added to the zirconia solid electrolyte powder, and a known method such as extrusion molding, isostatic pressing (rubber press), or press forming is used to form the solid electrolyte shown in FIG.
A hollow cylindrical molded body 27 having both open ends as shown in FIG.

(2) Patterns 28 and 29 to be the reference electrode 3 and the measurement electrode 4 are formed on the inner surface and the outer surface of the cylindrical molded body 27 by using a conductive paste as described above by a slurry dipping method and a screen. It is formed by printing, pad printing, roll transfer, brush painting, or the like. At this time, in printing the reference electrode 28 on the inner surface of the cylindrical molded body 27, it is efficient to fill the inside of the cylindrical molded body 27 with the conductive paste, discharge the paste, and apply the conductive paste to the entire inner surface.

Thereafter, a slurry in which the zirconia material is dispersed is poured into a predetermined depth from the end on the tip side of the cylindrical molded body 27, dried, and one end is sealed. Thereafter, the tip of the cylindrical molded body 27 is processed into a predetermined shape such as an arc. Thus, the sensor body A is manufactured.

(3) Next, a heater element B as shown in FIG. 4B is formed. First, using a slurry containing the above-mentioned zirconia powder, 50 to 500 μm, particularly 100 μm.
A green sheet 33 made of zirconia for forming a ceramic insulating layer having a thickness of about 300 μm is prepared.
Thereafter, on the surface of the green sheet 33, using a slurry obtained by appropriately adding an organic binder for molding to a ceramic powder such as alumina, spinel, forsterite, zirconia, or glass, a screen printing method, a pad printing method,
After printing by roll transfer method etc., screen printing method of conductive paste containing metal powder such as platinum on the surface,
The heating resistor pattern 34 including the lead pattern is formed by printing by a pad printing method, a roll transfer method, or the like. Then, the slurry made of the ceramic insulator is applied again. Thereafter, by forming the opening 35 by punching or the like, a heater element B composed of a laminate of the green sheet 33, the heating resistor pattern 34, and the green sheet 36 is obtained.

(4) Next, as shown in FIG. 4 (c), the heater element B is wound around the surface of the cylindrical sensor element A with an adhesive such as acrylic resin interposed therebetween. A laminate is produced. Note that the seam of the wound heater element B may overlap the sheet end portions, or may be bonded at a predetermined interval.

(5) Then, the above cylindrical laminate is placed in an inert atmosphere such as argon gas or the
By firing at 〜1700 ° C. for about 1 to 10 hours, the sensor element A and the heater element B can be fired simultaneously.

Further, in order to form the ceramic protective layer 13 shown in FIG. 1, after firing, a powder of alumina, spinel, zirconia or the like is printed and applied by a sol-gel method, a slurry dipping method, a printing method, or the like, and is baked. It is also possible to form by coating the above ceramics by a sputtering method or a plasma spraying method, or to apply a slurry for forming a ceramic protective layer 13 in advance when producing a cylindrical laminated body and then sinter it at the same time. It is.

According to the above-described manufacturing method, an integrated body of the sensor, the heater and the ceramic member can be manufactured in one firing step, and a separate bonding step is not required, so that the manufacturing yield and the manufacturing cost can be reduced. Very preferred because it can be achieved.

Although an example of the oxygen sensor according to the present invention has been described above, the present invention is not limited to the oxygen sensor shown in FIG. It goes without saying that any oxygen sensor having electrodes can be applied. For example, A
/ F sensor and a flat plate type oxygen sensor.

[0056]

EXAMPLE A commercially available 99.8% pure alumina powder and 5
Zirconia powder containing mol% Y ₂ O ₃ and an average particle size of about 1
A cermet powder of a platinum powder of μm and platinum or platinum / rhodium having an average particle size of 1 μm and containing 0 to 54.3% by volume of zirconia was prepared.

First, a clay is prepared by adding a polyvinyl alcohol solution to zirconia powder containing 5 mol% Y ₂ O _3, and has an outer diameter of about 4 mm after sintering by extrusion and an inner diameter of 2 mm.
mm, a cylindrical molded body whose both ends are opened is made up of 95% by weight of cermet powder and 5% by weight of terpineol and ethyl cellulose as organic resin components.
Using a conductive paste having a viscosity of about 0.4 Pas by weight, the paste is sprayed by a sprayer to form rectangular measurement electrode patterns and lead patterns having various thicknesses, and at the same time, over the entire inner surface of the cylindrical molded body. Also, the conductive paste was applied to form a reference electrode pattern.

Further, the tip of the cylindrical molded body was immersed in a slurry in which 20% by weight of mineral spirit was added to 100% by weight of zirconia powder, dried, and one end was sealed. A cylindrical sensor body A was produced.

A slurry was prepared by adding a polyvinyl alcohol solution to zirconia powder containing 5 mol% of Y ₂ O _{3, and} a green sheet having a thickness of about 200 μm was prepared.
A rectangular opening corresponding to the shape of the measurement electrode pattern was opened in the green sheet by punching.
Then, after applying a paste containing alumina powder to a thickness of about 20 μm on the surface of the zirconia green sheet having an opening, printing and coating the surface of the green sheet using a conductive paste containing platinum powder having an average particle diameter of 1 μm. Thus, a heating resistor pattern was formed. At this time, the width and thickness of the heating resistor pattern were set such that the average line width after sintering was 0.25.
mm, and the average maximum thickness was 20 μm. Then, a paste containing the above-mentioned alumina powder is applied again on the heating resistor pattern with a thickness of about 20 μm, and the heating resistor pattern is buried between alumina paste layers on the zirconia green sheet. The heater element B having the structure shown in FIG.

Next, the heater element B was wound around the surface of the sensor element A using an acrylic resin as an adhesive to form a cylindrical laminate. Thereafter, the cylindrical laminate was fired in the air at 1500 ° C. for 2 hours, and fired and integrated. At this time, the winding position of the heater element B was set such that the winding position of the heater element B from the end face of the sensor element A was 2 mm after firing.

Thereafter, a ceramic protective layer of spinel having a porosity of about 30% and a thickness of 100 μm is formed on the surface of the measurement electrode in the opening by plasma spraying to produce a stoichiometric air-fuel ratio sensor as shown in FIG. did.

(Evaluation) For the manufactured theoretical air-fuel ratio sensor, when the section along the longest diameter (that is, the diagonal line of the rectangle) in the plane of the measurement electrode was observed using a scanning electron microscope, the thickness was 6 μm or less. The total length was measured, and the ratio to the total length of the diagonal line was determined.

The sheet resistance of the measuring electrode was measured by measuring the resistance between both ends X and Y of the diagonal line, and dividing by the area of the measuring electrode to obtain the sheet resistance. The results are shown in Table 1.

Further, the prepared oxygen sensor element was subjected to CO, CO ₂ , H ₂ , N ₂ , C ₃ H _{8 at} 600 ° C.
And the gas response time of the element when the air-fuel ratio was instantaneously changed from 14 to 15 was determined. The measurement principle is
As shown in FIG. 5, when the air-fuel ratio is changed from 14 to 15 at the point w, the electromotive force sharply decreases. When the electromotive force difference is set to 100%, the electromotive force difference becomes 60% in the present invention. The time to point n was defined as the gas response time. The results are shown in Table 1.

For comparison, the same measurement was carried out for a commercially available flat plate type heater-integrated oxygen sensor (sample No. 27). Furthermore, in forming the measurement electrode,
In place of the cermet powder, a conductive paste was prepared using a mixed powder obtained by mixing a platinum powder having an average particle size of 1 μm and a zirconia powder having an average particle size of 1 μm at a volume ratio of 70:30, A stoichiometric air-fuel ratio sensor (sample No. 2) is formed in exactly the same manner except that the sensor is formed by a screen printing method.
8) was produced.

[0066]

[Table 1]

From the results shown in Table 1, when the cross section along the longest diameter in the plane of the measurement electrode is observed, the sample whose thickness is 6 μm or less is 50% or more of the total length of the longest diameter, and the sheet resistance is 0% It can be seen that the gas response time of each of the samples having a resistance of 0.3 to 6 Ω / mm ² is as extremely short as 200 ms or less. In contrast, the sample thickness is less than a length of 6μm less than 50% of the longest diameter full-length, and an area resistance of less than 0.3 [Omega / mm ^2, a sample of more than 6 [Omega / mm ² are both gas response The time was longer than 200 ms.

[0068]

As described in detail above, according to the oxygen sensor of the present invention, the measuring electrode is formed of a platinum group metal and zirconia, the sheet resistance is 0.3 to 6 Ω / mm ² , and the measuring electrode is The gas responsiveness can be improved by setting the length of the section having a thickness of 6 μm or less when observing a cross section along the longest diameter in a plane to be 50% or more of the total length of the longest diameter.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view illustrating an example of a heater-integrated oxygen sensor according to the present invention.

FIG. 2 is a sectional view taken along line AA in the oxygen sensor of FIG.

FIG. 3 is a partially cutaway sectional view of a measurement electrode.

FIG. 4 is a process chart for explaining an example of a method for manufacturing a gas sensor of the present invention.

FIG. 5 is a diagram for explaining the principle of a method for measuring a gas response time.

FIG. 6 is a sectional view showing a conventional oxygen sensor.

[Description of Signs] 1 Oxygen sensor 2 Cylindrical tube (zirconia solid electrolyte) 3 Reference electrode 4 Measurement electrode

Claims (4)

[The claims] 1. An oxygen sensor comprising a zirconia solid electrolyte substrate and a measurement electrode and a reference electrode provided on opposing surfaces of the zirconia solid electrolyte substrate, wherein the measurement electrode contains a platinum group metal and zirconia, and has a sheet resistance of 0. 3-6Ω / mm
An oxygen sensor, which is ² . 2. The apparatus according to claim 1, wherein a total length of the measurement electrode having a thickness of 6 μm or less when observed in a cross section along the longest diameter in a plane of the measurement electrode is 50% or more of the total length of the longest diameter. Oxygen sensor. 3. The measurement electrode according to claim 1, wherein the measurement electrode is formed by applying a paste of a cermet powder containing a platinum group metal and zirconia to the surface of a solid electrolyte and firing it. Oxygen sensor. 4. The oxygen sensor according to claim 1, wherein the ceramic insulating layer in which the heating resistor is embedded is formed integrally.