JP4409581B2 - Oxygen sensor element - Google Patents

Description

  The present invention relates to an oxygen sensor element for controlling the ratio of air and fuel in an internal combustion engine such as an automobile.

  Currently, in an internal combustion engine such as an automobile, by detecting the oxygen concentration in the exhaust gas and controlling the amount of air and fuel supplied to the internal combustion engine based on the detected value, harmful substances from the internal combustion engine, For example, a method of reducing CO, HC, and NOx is adopted.

As this detection element, a solid electrolyte type oxygen mainly composed of a solid electrolyte mainly composed of zirconia having oxygen ion conductivity and having a pair of electrode layers formed on the outer surface and the inner surface of a cylindrical tube sealed at one end. A sensor is used. A typical example of this oxygen sensor is a ZrO ₂ solid electrolyte. On the inner surface of a cylindrical tube sealed at the tip, a reference electrode made of platinum as a sensor part and in contact with a reference gas such as air is provided. A measurement electrode is formed on the outer surface of the cylindrical tube to be in contact with a gas to be measured such as exhaust gas.

  In such an oxygen sensor, as a so-called theoretical air-fuel ratio sensor (λ sensor), which is generally used for controlling the ratio of air and fuel near 1, a ceramic protective layer is provided on the surface of the measurement electrode. Thus, an oxygen concentration difference generated on both sides of the cylindrical tube at a predetermined temperature is detected, and the air-fuel ratio of the engine intake system is controlled.

  In the above theoretical air-fuel ratio sensor, it is necessary to heat the sensor section to an operating temperature of about 700 ° C. Therefore, a rod heater is inserted inside the cylindrical tube to heat the sensor section to the operating temperature. ing.

  However, in recent years, exhaust gas regulations have been strengthened, and it has become necessary to detect CO, HC, and NOx immediately after the engine is started. In response to such a request, as described above, in the indirect heating type cylindrical oxygen sensor in which the heater is inserted into the cylindrical tube, the time required for the sensor unit to reach the activation temperature (hereinafter referred to as the activation time). )) Was slow, and there was a problem that exhaust gas regulations could not be fully met.

In recent years, as a method for avoiding this problem, as shown in FIG. 5, a reference electrode 52 and a measurement electrode 53 are provided on the inner and outer surfaces of a flat solid electrolyte substrate 51 having an air introduction hole 50, and at the same time, ceramic insulation is provided. A heater-integrated sensor in which a heating element 55 made of platinum is embedded in the body 54 and a ceramic protective layer 56 is formed on the surface of the measurement electrode 53 has been proposed.
JP-A-5-256816

  However, the oxygen sensor element integrated with this heater is different from the conventional indirect heating method in that it is a direct heating method. Since it was fabricated by simultaneous firing at a high temperature, there was a big problem that the gas responsiveness of the measurement electrode was poor. One reason for this is that when the measurement electrode is printed and coated with a conductive paste and then co-fired with the solid electrolyte, the measurement electrode becomes dense during firing due to the high firing temperature. There is a problem that the diffusion rate is reduced and the gas responsiveness is deteriorated.

  In order to suppress such densification of the measurement electrode, it is conceivable to contain a large amount of ceramic particles in the measurement electrode. However, if the content of the ceramic particles is large, there is a problem that gas responsiveness deteriorates. It was.

  Accordingly, an object of the present invention is to provide an oxygen sensor element that suppresses densification of measurement electrodes and has excellent gas responsiveness.

As a result of studying the above problems, the present inventors can suppress densification of the measurement electrode during firing by changing the blending ratio of the ceramic particles in the measurement electrode. The inventors have found that the porosity is increased and the gas responsiveness is improved, and the present invention has been achieved.

That is, the oxygen sensor element of the present invention is a sintered body of a composite material in which a zirconia solid electrolyte substrate having a first surface and a second surface facing the first surface, and metal particles and ceramic particles are mixed, A measurement electrode formed on the first surface by co-firing with the zirconia solid electrolyte substrate ; a ceramic protective layer formed on the surface of the measurement electrode by co-firing with the measurement electrode; An oxygen sensor comprising a reference electrode formed on the second surface, wherein the ceramic protective layer side of the measurement electrode has a higher content of the ceramic particles than the first surface side.

It is desirable that a ceramic protective layer is formed on the surface of the measurement electrode.

As described above in detail, according to the present invention, the measurement electrode is formed of a mixed material with metal particles and ceramic particles, and the content of the ceramic particles on the surface side of the measurement electrode is set to be higher than that on the first surface side. By increasing the number, it is possible to provide an oxygen sensor element excellent in gas responsiveness while suppressing densification of the surface of the measurement electrode in contact with the measurement gas and preventing poisoning of the measurement electrode.

  FIG. 1A is a schematic plan view showing an example of the basic structure of the oxygen sensor of the present invention, and FIG. 1B is a sectional view taken along line xx in FIG. This oxygen sensor element is generally called a theoretical air twist ratio sensor element.

  In FIG. 1, an oxygen sensor element 1 includes a solid electrolyte substrate 2 made of zirconia and having oxygen ion conductivity, and a reference electrode 4 and a measurement electrode 5 that are in contact with air on the opposing surfaces of the solid electrolyte substrate 2. It is formed and forms a sensor part for detecting the oxygen concentration.

  That is, the solid electrolyte substrate 2 has a flat hollow shape with a sealed end, and the hollow portion forms the air introduction hole 3. A reference electrode 4 in contact with a reference gas such as air is deposited on the inner wall of the air introduction hole 3, and a gas to be measured such as exhaust gas is formed on the outer surface of the solid electrolyte substrate 2 facing the reference electrode 4. A measurement electrode 5 is formed in contact therewith. Further, from the viewpoint of preventing the electrode from being poisoned by exhaust gas, a ceramic protective layer 6 made of a porous material is formed on the surface of the measurement electrode 5.

  Further, leads 7 a and electrode pads 7 b connected to the measurement electrode 5 are further formed on the surface of the solid electrolyte substrate 2.

  Further, in the oxygen sensor element 1 of the present invention, the heater part in which the heating element 9 is embedded in the ceramic insulating layer 8 is formed integrally with the sensor part. This heater part is formed by simultaneous firing with the sensor part.

According to the present invention, in such an oxygen sensor element 1, the measuring electrode 5, and the metal particles 10 and the ceramic particles 11 is formed by a composite material mixed.

Therefore, enlarged views of structural examples of the measurement electrode 5 are shown in FIGS. In the example of FIG. 2, the mixing ratio of the metal particles 10 and the ceramic particles 11 is different between the zirconia solid electrolyte substrate 2 side of the measurement electrode 5 and the surface side, and the ceramic particle content on the surface side is different from that of the substrate 2 side. Has also increased.

According to the present invention, effects such as by increasing remote by the solid electrolyte substrate 2 side the content of the ceramic particles definitive on the surface side, thereby preventing the densification of the measuring electrode, enhancing the poisoning prevention effect due to the electrode itself There is an effect.

On to effectively exhibit the effect described above, the metal particles: the ratio of the ceramic particles solid electrolyte substrate 2 side 40: 60 to 60: 40, in particular 45: 55 to 55: 45, on the surface side 35: 65 5:95, particularly 25:75 to 15:85 is desirable. In addition, this ratio is an area ratio of the metal particles 10 and the ceramic particles 11 when the total area of the metal particles 10 and the ceramic particles 11 is 100 when an electron micrograph of a longitudinal section is observed. It was substantially the same as the mixing ratio of each particle in the volume ratio.

  Further, the metal particles 10 forming the measurement electrode 5 have an average minor axis diameter of 0.3 to 1.3 μm in the cross section, and the ceramic particles 11 have an average minor axis diameter of 0.2 to 1.5 μm in the section. Appropriately present in size.

In the present invention, as shown in FIG. 3, an intermediate layer c containing a larger amount of ceramic particles than the electrode layer a and the electrode layer b is interposed between the electrode layer a and the electrode layer b. Gas responsiveness can be improved. In this case, the thickness of the intermediate layer c is 1 to 100 μm, particularly 20 to 50 μm, and the content of ceramic particles in the intermediate layer c is suitably 80% or more.

The reference electrode 6, metallic particles: ceramic particles, 60: 40 to 90: it is desirable that a mixture with 10 ratio.

In the present invention, the metal particles 10 forming the measurement electrode 5 and the reference electrode 4 are made of at least one of Pt, Rh, Pd, and Au, particularly platinum, or platinum, and a group of rhodium, palladium, ruthenium, and gold. An alloy with one selected is used. The ceramic particles 11 are preferably made of ceramics mainly composed of zirconia or alumina. In particular, zirconia containing at least one rare earth element (RE) of Y, Yb, Sc, and Nd in terms of RE ₂ O _{3 in} terms of RE ₂ O _{3 is} particularly 6 to 10 mol% in terms of corrosion resistance and high strength. desirable.

  This ceramic particle 11 is used for the purpose of preventing metal grain growth in the electrode during sensor operation, and for the purpose of increasing the contact at the so-called three-phase interface between the metal particles, the solid electrolyte, and the gas related to responsiveness. When the amount of ceramic particles is increased, the poisoning prevention effect is obtained.

  Further, the electrode shape may be a quadrangle or an ellipse. The thickness of the electrode is preferably 3 to 20 μm, particularly preferably 5 to 10 μm.

  On the other hand, the ceramic insulating layer 8 in which the heating element 9 is embedded is preferably composed of dense ceramics having a relative density of 80% or more and an open porosity of 5% or less. The ceramic insulating layer 8 may be made of at least one selected from the group consisting of alumina ceramics, ceramics mainly composed of composite oxides of Al and Mg, and ceramics mainly composed of composite oxides of Al and rare earth elements. It is desirable in terms of corrosion resistance and high strength.

  Further, the ceramic insulating layer 8 may contain 1 to 10% by weight of Mg, Ca, and Si in total for the purpose of improving sinterability. Is preferably controlled to 50 ppm or less in terms of oxide in order to migrate and deteriorate the electrical insulation of the heater substrate. In addition, by setting the relative density within the above range, the substrate strength increases, and as a result, the mechanical strength of the oxygen sensor itself can be increased. The thickness of the ceramic insulating layer 8 is suitably 5 to 20 μm.

Further, the heating element 9 is formed with platinum in a thickness of 5 to 50 μm to enhance the co-sinterability with the ceramic insulating layer 8, but in some cases, the heating element 9 is selected from the group consisting of platinum, rhodium, palladium and ruthenium. One kind of alloy can be used. As a pattern of the heating element, not only a meander (wave shape) structure as shown in FIG. 2 but also a U-shaped structure extending in the longitudinal direction and folded at the end in the longitudinal direction may be used.

The solid electrolyte used in the oxygen sensor element of the present invention is made of a ceramic containing ZrO ₂ , and Y ₂ O ₃ and Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O are used as stabilizers. _3, Dy ₂ O ₃ or the like 1 to 30 mol% of rare earth oxide in terms of oxide, preferably the partially stabilized ZrO ₂ or stabilized ZrO ₂ containing 3 to 15 mol% are used. In addition, for the purpose of improving the sinterability, Al ₂ O ₃ and SiO ₂ can be added to the ZrO ₂ , but if it is contained in a large amount, the creep properties at high temperatures deteriorate, The total amount of Al ₂ O ₃ and SiO ₂ is preferably 5% by weight or less, particularly 2% by weight or less.

  Next, the manufacturing method of the oxygen sensor element of the present invention will be specifically described with reference to FIG. 4, taking the oxygen sensor element of FIGS. 1 and 3 as an example.

  First, as shown in FIG. 4, a zirconia green sheet 20 is prepared. The green sheet 20 is formed by adding a forming organic binder as appropriate to a ceramic solid electrolyte powder having oxygen ion conductivity of zirconia, and using a doctor blade method, extrusion molding, isostatic pressing (rubber press) or press. It is produced by a known method such as formation.

  Next, the measurement electrode pattern 21, the reference electrode pattern 22, the lead pattern 23, the electrode pad 24, the through-hole 25, and the like are formed on both surfaces of the green sheet 20, respectively, for example, with an average particle size of 0.3 to 1.. A metal powder such as 3 μm of platinum and a ceramic powder such as zirconia or alumina having an average particle diameter of 0.2 to 1.5 μm are mixed in the ratio as described above. Then, three types of conductive paste are prepared by mixing at different ratios of the three compositions forming the measurement electrode. The ratio at this time is prepared by using the area ratio of each layer as a volume ratio.

  Then, in order to form the electrode layers a and b and the intermediate layer c on the surface of the green sheet 20, three types of conductive paste are sequentially printed and applied to form the three layers 21a, 21b and 21c.

  Next, on the side of the green sheet 20 on which the reference electrode pattern 22 is formed, the green sheet 28 in which the air introduction hole 27 is formed and the green sheet 29 in which the lower wall of the air introduction hole 27 is formed are replaced with an acrylic resin or an organic solvent. The sensor laminate A is produced by interposing an adhesive such as the above or by mechanically adhering while applying pressure with a roller or the like.

  Next, a ceramic green sheet 31 a made of alumina, for example, is formed on the surface of the zirconia green sheet 30. Thereafter, the heating element 32 and the lead portion 33 are formed on the surface of the ceramic green sheet 31a by a slurry dip method, or screen printing, pad printing, or roll transfer using a conductive paste containing platinum, and further on this surface. Once again, an alumina ceramic green sheet 31b is formed.

  Further, instead of laminating the green sheets 31a and 31b, a paste made of alumina powder may be printed by a slurry dip method, screen printing, pad printing, or roll transfer.

  An electrode pad 34 is provided on the back surface of the green sheet 30 and is connected to the electrode pad 34 through a lead portion 33 and a through hole 35 formed in the green sheet 30 or 31a.

  Next, the sensor laminate A and the heater laminate B are bonded by interposing an adhesive such as an acrylic resin or an organic solvent, or by mechanically bonding the two while applying pressure with a roller or the like. To produce a laminate.

  Thereafter, the laminate is fired at 1300 ° C. to 1700 ° C. for 1 to 10 hours in the air or in an inert gas atmosphere. At this time, in order to suppress warping during firing, the amount of warpage can be reduced by placing a smooth substrate such as alumina on the laminate as a weight.

  If necessary, a ceramic protective layer such as zirconia containing a predetermined amount of a pore-forming agent composed of organic particles or carbon particles that evaporates or burns away after sintering is formed on the surface of the measurement electrode. Ceramic protection is achieved by using a slurry containing a ceramic composition for the above purpose, printing a predetermined thickness on the surface of the measurement electrode 21 by a slurry dipping method, or screen printing, pad printing, or roll transfer, and then firing simultaneously with the above firing. Layer 26 can be formed. The ceramic protective layer can also be formed by firing simultaneously with the measurement electrode and the solid electrolyte.

  A theoretical air-fuel ratio sensor element integrated with the heater shown in FIG. 1 was produced based on FIG.

First, a zirconia green sheet in which a polyvinyl alcohol solution is added to zirconia powder containing 5 mol% of Y ₂ O ₃ to prepare a slurry, and after sintering by extrusion molding, the thickness becomes 0.4 mm. 20 was produced.

The surface of the green sheet 20 has an average particle size of 0. 1% containing platinum powder having an average particle size of 0.5 µm, alumina powder having an average particle size of 0.8 µm, or 8 mol% of Y ₂ O ₃ . Platinum paste having a different composition by mixing 5 μm zirconia coprecipitation raw material powders in the proportions shown in Table 1, adding acrylic resin as a binder to this mixture, and mixing using TPO as a solvent Were prepared from the surface of the green sheet 20 in the order shown in Table 1.

  Further, a platinum paste prepared by mixing the reference electrode pattern 22, the lead pattern 23, and the electrode pad 24 at a ratio of 80% by volume of the platinum powder and 20% by volume of the zirconia powder and having a thickness after firing of 10 μm. It was applied by printing. Further, through holes 25 were formed at predetermined locations on the green sheet 20 and filled with the platinum paste.

  Next, the green sheet 28 having a thickness of 600 μm and the green sheet 29 having a thickness of 400 μm in which the air introduction hole 27 was formed were laminated with an adhesive of an acrylic resin to obtain a laminate A for a sensor.

  Next, an acrylic resin is added as an organic binder to an alumina powder having an average particle size of 0.5 μm on the surface of a 400 μm thick zirconia green sheet 30 to prepare an alumina slurry, and this slurry is fired by screen printing to a thickness of 10 μm. The ceramic insulating layer 31a was formed by coating. Thereafter, using a platinum paste containing 20% by volume of alumina powder having an average particle size of 0.5 μm and 80% by volume of platinum powder, the heating element pattern 32 is formed by screen printing so that the thickness after firing is about 15 μm. A lead pattern 33 was formed. And the ceramic insulating layer 31b was formed so that it might become 10 micrometers after baking again using the alumina slurry on the surface, and the laminated body B for heaters was produced.

  Thereafter, the sensor laminate A and the heater laminate B were joined together with an adhesive to obtain a heater integrated sensor element laminate.

  After that, as a ceramic protective layer, a slurry in which resin beads having a size of about 10 μm are added to 8 mol% yttria-containing zirconia powder is prepared, and the thickness after firing is measured on the surface of the measurement electrode pattern by screen printing. It was formed with a thickness of 100 μm.

  Thereafter, the laminate was baked at a temperature of 1500 ° C. for 1 hour to produce an oxygen sensor element integrated with a heater.

  The gas responsiveness was evaluated with respect to the produced oxygen sensor element. In the evaluation, the oxygen sensor element is incorporated in a metallic housing, and the exhaust gas air-fuel ratio is set to 14 while maintaining the exhaust gas temperature at 350 ° C. by an engine control computer outside the 1500 cc four-cylinder gasoline engine. Necessary for sensor element output to change from 0.6V to 0.3V and from 0.3V to 0.6V when switching from .0 to 15.4 and vice versa from 15.4 to 14.0 Response time TRL and TLR were measured respectively. The measurement time was measured twice, 100 hours and 500 hours after starting the engine. At this time, a commercially available heater-integrated sensor element was also used for measurement for comparison. The results are shown in Table 1.

At this time, the cross section of the measurement electrode was photographed at five locations using a scanning electron microscope (3000 times), and the ratio of the total area of the pores to the sectional area of the electrode layers a, b, and c determined from the photographs was calculated as the porosity. Calculated as In addition, regarding the thickness of each layer of the measurement electrode, five cross-sectional photographs were similarly taken with a scanning electron microscope (300 to 1000 times), the thicknesses at five locations were measured per cross-section, and the average of these was measured. The thickness was taken.

From Table 1 , the ceramic ratio on the conventional product (No. 27) and the solid electrolyte substrate side is larger than the ceramic ratio on the surface layer side. In the case of 12, in the gas responsiveness, the response time exceeded 150 ms, and the responsiveness was very poor.

In contrast, on the basis of the present invention, the content of the ceramic particles definitive on the surface side showing a 150ms following good characteristics even after 500 hours had passed by many remote I solid electrolyte substrate.

Among them, when the solid electrolyte substrate side is metal particles: ceramic particles = 40: 60 to 60:40 and the surface side is metal particles: ceramic particles = 35: 65 to 5:95, all the samples of the present invention have a gas response. And the poisoning prevention effect of the measurement electrode in a long-time operation was large, the deterioration of gas responsiveness was small, and an excellent characteristic of 110 ms or less was exhibited even after 500 hours. Further, by providing the intermediate layer c, the gas responsiveness could be further improved.

  In this example, the thickness of the electrode layer a was 5 μm, the electrode layer b was 10 μm, and the intermediate layer c was 10 μm.

Example 2
Sample No. 1 in Table 1 in Example 1 was used. 5 and sample no. In the measurement electrode having the two-layer or three-layer structure shown in FIG. 18, the gas responsiveness was evaluated in the same manner as in Example 1 by changing the thicknesses of the electrode layers a, b, and c.

  As a result, when the electrode layer a is in the range of 1 to 10 μm, the electrode layer b is in the range of 3 to 20 μm, and the electrode layer c is in the range of 1 to 100 μm, the gas responsiveness and the effect of preventing the poisoning of the electrode during long-time operation are great, The deterioration of the property was small, and excellent characteristics of 110 ms or less were exhibited even after 500 hours.

An example of the basic structure of the oxygen sensor of the present invention is shown in (a) a schematic plan view and (b) an xx cross-sectional view in (a). It is an expanded sectional view of the measurement electrode of the oxygen sensor element of the present invention. It is an expanded sectional view of the other measuring electrode of the oxygen sensor element of the present invention. It is a disassembled perspective view for demonstrating the manufacturing method of the oxygen sensor element of this invention of FIG. The schematic perspective view for demonstrating the structure of the other conventional flat type heater integrated oxygen sensor element was shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Oxygen sensor element 2 Solid electrolyte substrate 3 Atmospheric introduction hole 4 Reference electrode 5 Measurement electrode 6 Ceramic protective layer 8 Ceramic insulating layer 9 Heating element 10 Metal particle 11 Ceramic particle a, b Electrode layer c Intermediate layer

Claims (3)

A zirconia solid electrolyte substrate having a first surface and a second surface facing the first surface;
A sintered body of a composite material in which metal particles and ceramic particles are mixed, and a measurement electrode formed on the first surface by simultaneous firing with the zirconia solid electrolyte substrate;
On the surface of the measurement electrode, a ceramic protective layer formed by co-firing with the measurement electrode,
An oxygen sensor comprising a reference electrode formed on the second surface,
The oxygen sensor element, wherein the ceramic protective layer side of the measurement electrode has a higher content of the ceramic particles than the first surface side. The oxygen sensor element according to claim 1 , wherein the ceramic protective layer is fired simultaneously with the measurement electrode and the zirconia solid electrolyte substrate. Metal particles in the measuring electrode, Pt, Rh, Pd, consisting at least one or more kinds of Au, the ceramic particles, claim, characterized by comprising a ceramic mainly composed of zirconia or alumina 1 or claim 2 oxygen sensor element according to.

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