JP2006284223A - STRUCTURE OF ELECTRODE PART FOR NOx MEASURING, ITS FORMING METHOD AND NOx SENSOR ELEMENT - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a structure of an electrode part for NOx measuring capable of ensuring durable reliability over a long period of time. <P>SOLUTION: In the NOx measuring electrode part structure having a measuring electrode, which comprises cerment of an electrode metal material capable of reducing or decomposing an NOx component in a gas to be measured and a ceramic material, and an electrode protecting layer, which comprises a porous ceramic layer, provided on the measuring electrode so as to cover the measuring electrode formed thereto by screen printing, the electrode protecting layer is formed in a form showing an almost trapezoidal cross-sectional shape having at least the flat upper bottom part extending in a horizontal direction on the measuring electrode and height change parts respectively gradually reduced in height toward the lateral part of the measuring electrode from both ends of the upper bottom part. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an improved NOx measurement electrode part structure, a method of forming the same, and a NOx sensor element. In particular, in a NOx measurement electrode part structure composed of a measurement electrode and an electrode protective layer covering the measurement electrode, the durability and reliability thereof. Further, the present invention relates to an electrochemical cell and a NOx sensor element configured by using such a NOx measurement electrode part structure.

Conventionally, various measurement methods and apparatuses have been proposed in order to know the concentration of NOx components such as NO and NO _{2 in} a measured gas such as an automobile exhaust gas. A measurement electrode made of a cermet (sintered body) of an electrode metal material and a ceramic material capable of reducing or decomposing NOx components in the measurement gas, and a porous material formed on the measurement electrode so as to cover it It includes an electrochemical cell formed by forming a NOx measurement electrode part structure composed of an electrode protective layer made of a ceramic layer on a predetermined solid electrolyte body. With such a measurement electrode, the NOx component is reduced or reduced. The NOx concentration in the gas to be measured is determined by decomposing and measuring the amount of oxygen generated by the reduction or decomposition of the NOx component. Sa element, furthermore, NOx sensor having such devices has been demonstrated.

  For example, in JP-A-8-271476 (Patent Document 1), JP-A-11-237362 (Patent Document 2), JP-A 2000-28576 (Patent Document 3), etc., a solid electrolyte having a predetermined thickness is used. A buffer space and first and second cavities are provided inside the element body (solid electrolyte body) having a laminated structure in which a plurality of layers are laminated and integrated, and a gas for introducing an external measurement gas Between the inlet and the buffer space, between the buffer space and the first space, and between the first space and the second space, respectively. In addition, there are provided first and second oxygen pump means for controlling the oxygen partial pressure in the first and second cavities, respectively, and in addition, in the second vacant space, The above-mentioned NOx measurement electrode part structure is formed, and an external measurement gas is supplied from the gas inlet to a predetermined Under the diffusion resistance, the buffer space, the first space and the second cavity is adapted to be brought successively introduced. In the first and second cavities, the oxygen partial pressure (concentration) in the introduced gas to be measured is controlled by the operation of the corresponding oxygen pump means. The structure is such that the NOx component present in the atmosphere can be brought into contact with the measurement electrode by being controlled to an oxygen partial pressure that can be reduced or decomposed by the measurement electrode.

  By the way, since the NOx sensor element having such a structure is generally manufactured in a laminated structure, elements such as necessary electrodes and leads thereof, a protective layer and a heater layer are generally laminated. The solid electrolyte layer is formed by a screen printing technique, and is formed in the same manner even in the measurement electrode disposed in the second space. That is, an electrode paste for providing a measurement electrode is printed at a predetermined position in a portion where a second void of the laminated solid electrolyte layer is formed by pattern printing by a normal screen printing operation using a mesh screen. A measurement electrode pattern is formed on the solid electrolyte layer, and a ceramic paste for providing an electrode protection layer is further printed thereon by a similar screen printing operation. It is formed so as to cover. After that, a perforated solid electrolyte layer formed by punching and forming a void corresponding to the second void and a solid electrolyte layer covering the void are sequentially laminated to form a laminated structure, and then fired and integrated. As a result, the target NOx sensor element is formed, and at the same time, the NOx measurement electrode part structure comprising the measurement electrode and the electrode protective layer provided so as to cover it is formed in the second space. It is like that.

  In the NOx sensor element obtained in this way, the electrode protective layer constituting the NOx measurement electrode portion structure disposed in the second space is a harmful component present in the atmosphere in the second space. However, when such an electrode protection layer is formed by the screen printing technique as described above, the electrode protection layer is formed on the measurement electrode by a dome. It becomes a coating layer having an arc-shaped cross section that exhibits the outer surface (upper surface) of a mold or a “manju” type, and the electrode protective layer portion located near the end of the measurement electrode has a height at the center of the measurement electrode It is lower than the electrode protective layer portion.

  However, in the obtained NOx sensor element, when it is used for NOx measurement of a gas to be measured such as exhaust gas, the second measurement is performed due to repeated measurement or temperature change of the gas to be measured. The electrode metal material of the measurement electrode in the NOx measurement electrode portion structure provided in the void repeatedly oxidizes and reduces, and the generation of stress due to this causes cracks in the electrode protective layer or peeling off of the measurement electrode. As a result, an increase or decrease in NOx sensitivity in the NOx sensor element is caused to change the NOx measurement accuracy, resulting in a problem that the sensor life is shortened.

Further, for the measurement electrode, a ZrO ₂ material is advantageously used as the ceramic material constituting the cermet in order to achieve a stronger integration with the solid electrolyte layer on which the measurement electrode is formed. This ZrO ₂ repeats the T (tetragonal system) / M (monoclinic system) transformation according to repeated heating and cooling corresponding to the repetition of the measurement operation, and is repeated over a long period of time. The strength of the electrode structure has been lowered, and this has the inherent risk of accelerating the phenomenon described above.

JP-A-8-271476 JP-A-11-237362 JP 2000-28576 A

  Here, the present invention has been made in view of such circumstances, and a problem to be solved by the present invention is to provide a NOx measurement electrode part structure capable of ensuring long-term durability and reliability. Another object of the present invention is to provide a method capable of advantageously forming such a NOx measurement electrode part structure, and further to provide a NOx sensor element having improved durability by adopting such a NOx measurement electrode part structure. There is also.

  In the present invention, in order to solve the problems related to the NOx measurement electrode part structure as described above, the cermet of the electrode metal material and the ceramic material capable of reducing or decomposing the NOx component in the measurement gas is used. In the NOx measurement electrode part structure formed by screen printing, the electrode protective layer formed on the measurement electrode and the electrode protective layer made of a porous ceramic layer provided so as to cover the measurement electrode. A substantially trapezoidal cross-sectional shape having at least a flat upper base portion extending horizontally in the horizontal direction on the measurement electrode and a height changing portion whose height gradually decreases from both ends of the upper base portion to the sides of the measurement electrode. In this embodiment, the NOx measurement electrode part structure is characterized by being formed.

  Further, in order to solve the above-described problem relating to the method for forming the NOx measurement electrode part structure, the present invention uses the method for forming the NOx measurement electrode part structure as described above, and the electrode protective layer is disposed on the measurement electrode. Further, the gist of the method is a method for forming a NOx measurement electrode portion structure, which includes a step of forming by a screen printing technique using a metal mask.

  Furthermore, in the present invention, in order to solve the above-described problems relating to the NOx sensor element, a measurement electrode comprising a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing NOx components in the gas to be measured. And a NOx measurement electrode part structure formed on the measurement electrode so as to cover the electrode protection layer made of a porous ceramic layer and covering the measurement electrode. The NOx component in the gas to be measured is measured by reducing or decomposing the NOx component at the measurement electrode and measuring the amount of oxygen generated by the reduction or decomposition of the NOx component. In the NOx sensor element for determining the concentration of NOx, the NOx measurement electrode part structure extends the electrode protective layer in the horizontal direction at least on the measurement electrode. NOx measurement electrode formed in a form that has a substantially trapezoidal cross-sectional shape having a flat upper base and a height change part in which the height gradually decreases from both ends of the upper base to the sides of the measurement electrode. The gist of the NOx sensor element is that it is configured by a partial structure.

  As described above, in the NOx measurement electrode portion structure according to the present invention, the electrode protection layer has a flat upper bottom portion extending in the horizontal direction on at least the measurement electrode, and a high height outward from the measurement electrode from both ends of the upper bottom portion. In other words, in the substantially trapezoidal cross-sectional shape having the height change portion where the upper surface height gradually decreases, the electrode protective layer is formed so as to cover the measurement electrode. Therefore, even if stress is generated by oxidation / reduction of the electrode metal material, the electrode protective layer located at the end of such a measurement electrode is formed. It is possible to effectively suppress or prevent the occurrence of cracks at the site, and this causes the measurement electrode itself to deteriorate or peel off due to the occurrence of such cracks. Are also effectively eliminated are such problems, than Te is the endurance reliability can be made to significantly improve long-term.

  Further, according to the method for forming the NOx measurement electrode portion structure according to the present invention described above, such a substantially trapezoidal electrode protective layer can be easily and easily formed on the measurement electrode by a screen printing operation using a metal mask. Thus, the practical value of such a NOx measurement electrode part structure can be advantageously increased.

  Further, the NOx sensor element according to the present invention described above is provided with an electrochemical cell provided with the NOx measurement electrode part structure with improved durability reliability as described above, so that the element life is effectively improved. It will be able to be enhanced, and will exhibit the characteristics that enable long-term use of the vehicle.

Aspects of the Invention

  By the way, the present invention can be suitably implemented in various aspects as listed below in order to solve the problems described above or the problems grasped from the description of the entire specification and the drawings. Each aspect described below can be employed in any combination. It should be noted that aspects or technical features of the present invention are not limited to those described below, and can be recognized based on the description of the entire specification and the inventive concept disclosed in the drawings. Should be understood.

(1) A measurement electrode composed of a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing a NOx component in a gas to be measured, and porous ceramic provided on the measurement electrode so as to cover it In the NOx measurement electrode part structure formed by screen printing, an electrode protection layer composed of a layer is formed of a flat upper bottom portion extending at least horizontally on the measurement electrode, and both ends of the upper bottom portion. A NOx measurement electrode part structure formed in a form having a substantially trapezoidal cross-sectional shape having a height changing part whose height gradually decreases from the side to the side of the measurement electrode.

(2) The above aspect (1), wherein the upper bottom portion in the trapezoidal cross-sectional shape of the electrode protective layer is protruded laterally from both end portions of the measurement electrode so as to be larger than the width of the measurement electrode. The NOx measurement electrode part structure described in 1.
According to such a configuration of the aspect (2), the operation and effect of the present invention relating to the above-described NOx measurement electrode part structure can be achieved more effectively.

(3) The said electrode protective layer is described in the said aspect (1) or (2) currently formed so that the width | variety of the upper bottom part of the trapezoid cross-sectional shape may be 1.1 times or more of the width | variety of the said measurement electrode. NOx measurement electrode part structure.
In addition, when the configuration of the aspect (3) is employed, the above-described operation and effect of the NOx measurement electrode part structure of the present invention can be achieved even more advantageously.

(4) The electrode protective layer has a thickness of 0.9 to 1 in the sum of the thickness of the measurement electrode and the thickness of the electrode protective layer on the side positions 80 μm apart from both ends of the measurement electrode. The NOx measurement electrode part structure according to any one of the above aspects (1) to (3), which has a thickness that is a value multiplied by 0.0.
Furthermore, according to the configuration of the aspect (4), an electrode protection layer having a substantially trapezoidal cross-sectional shape is formed on the measurement electrode so as to sufficiently cover the measurement electrode, thereby protecting the electrode. Thus, the effect of preventing cracking of the layer can be further effectively improved.

(5) The thickness of the measurement electrode is 15 to 35 μm, the thickness of the electrode protective layer formed on the measurement electrode is 20 to 45 μm, and the thickness of the measurement electrode and the thickness of the electrode protective layer The NOx measurement electrode part structure according to any one of the above aspects (1) to (4), wherein the sum is 70 μm or less.

(6) The NOx measurement electrode part according to any one of the above aspects (1) to (5), wherein the electrode protective layer is formed in a size obtained by adding 200 to 600 μm to the width of the measurement electrode. Construction.

(7) the one said electrode metal material that provides cermet constituting the measuring electrode is a noble metal, Y ₂ O ₃ stabilized the ceramic material provide such cermet comprises Y ₂ O ₃ of 5.5～8.5Mol% The NOx measurement electrode part structure according to any one of the above aspects (1) to (6), which is a modified ZrO ₂ material.
As described above, by using a Y ₂ O ₃ stabilized ZrO ₂ material containing a specific amount of Y ₂ O ₃ as a ceramic material for providing a cermet constituting the measurement electrode, the T / of ZrO ₂ caused by heating / cooling is used. It is possible to advantageously suppress or prevent the problem of deterioration of the measurement electrode and, consequently, peeling of the measurement electrode, based on the repetition of the M transformation.

(8) The electrode metal material that provides the cermet constituting the measurement electrode is a noble metal, and the noble metal is Pt or an alloy of Pt and Rh, and Pt: Rh = 100 to 40% by weight: 0 to 60 The NOx measurement electrode part structure according to any one of the above aspects (1) to (7), wherein Pt and Rh are used so as to satisfy the ratio by weight.

(9) The electrode metal material that provides the cermet constituting the measurement electrode is a noble metal, and the ratio (vol%) between the noble metal and the ceramic material is in a range of 65/35 to 40/60. The NOx measurement electrode part structure according to any one of the above aspects (1) to (8) that is used.

(10) From the solid electrolyte in which the measurement electrode and the electrode protective layer are integrally formed on a bottom surface in a void formed by being surrounded by a solid electrolyte, and partition and form the void The NOx measurement electrode part structure according to any one of the above aspects (1) to (9), which is disposed through a space between the side wall without contacting the side wall.
According to such an aspect (10), the measurement electrode and the electrode protective layer, particularly the electrode protective layer in the NOx measurement electrode part structure are separated from each other so as not to come into contact with the solid electrolyte body constituting the side wall in the second space. The interface peeling between the side wall surface and the electrode protection layer based on the difference in thermal shrinkage between the side wall surface and the material constituting the electrode protection layer, in addition to the peeling of the electrode protection layer due to it, It is possible to effectively avoid the problem of peeling of the measurement electrode.

(11) A measurement electrode composed of a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing a NOx component in a gas to be measured, and porous ceramic provided on the measurement electrode so as to cover it In the NOx measurement electrode portion structure formed by screen printing, the electrode protective layer comprising a layer, the ceramic material that provides the cermet while using a noble metal as the electrode metal material that provides the cermet constituting the measurement electrode A NOx measuring electrode part structure using a Y ₂ O ₃ stabilized ZrO ₂ material containing 5.5 to 8.5 mol% of Y ₂ O ₃ .
According to the configuration of the aspect (11), a cermet is formed by forming a cermet with a noble metal using a specific Y ₂ O ₃ stabilized ZrO ₂ material and forming a measurement electrode. Even when the ceramic material zirconia (ZrO ₂ ) causes the T / M transformation, the sufficient strength is ensured to prevent the electrode skeleton from becoming brittle, and problems such as peeling of the measurement electrode Can be advantageously prevented from occurring.

(12) In the method for forming the NOx measurement electrode part structure according to any one of the aspects (1) to (10), the electrode protective layer is formed on the measurement electrode, and a screen using a metal mask. A method for forming a NOx measurement electrode part structure, comprising a step of forming by a printing technique.

(13) An electrochemical cell characterized in that the NOx measurement electrode part structure according to any one of the aspects (1) to (11) is provided on a predetermined solid electrolyte body. .

(14) A measurement electrode composed of a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing NOx components in the gas to be measured, and porous ceramics provided on the measurement electrode so as to cover it An electrochemical cell formed by forming a NOx measurement electrode part structure composed of an electrode protective layer comprising a layer on a predetermined solid electrolyte body, and reducing or decomposing the NOx component at the measurement electrode In addition, by measuring the amount of oxygen generated by the reduction or decomposition of the NOx component, a NOx sensor element for determining the concentration of the NOx component in the gas to be measured is used, and the NOx measurement electrode part structure has the aspect described above. (1) thru | or (11) WHEREIN: It is comprised by the NOx measurement electrode part structure as described in any one, The NOx sensor element characterized by the above-mentioned.

(15) It has a longitudinal element shape having a laminated structure in which a plurality of solid electrolyte layers are laminated and integrated, and a buffer space and a first cavity are formed therein from the element tip side toward the element base side. And the second vacant space are separately disposed, and the gas to be measured externally passes through the first diffusion rate-limiting means from the gas inlet formed on the tip side of the element. And the atmosphere in the buffer space is introduced into the first space through the second diffusion rate-limiting means, and the atmosphere in the first space is further introduced into the first space through the third diffusion rate-limiting means. In the NOx sensor element configured to be guided into the second space, the NOx measurement electrode part structure according to any one of the aspects (1) to (11) is provided on the wall surface of the second space. NOx characterized by integrally forming Capacitors element.

(16) The first inner pump electrode and the first outer pump electrode formed inside and outside the first space, and oxygen contained in the atmosphere introduced from the buffer space, The NOx sensor element according to the above aspect (15), in which main pumping means for performing pumping processing based on a control voltage applied between the pump electrodes is disposed.

(17) The first inner pump electrode in the main pump means is formed on the ceiling electrode portion formed on the ceiling surface of the first cavity, the bottom electrode portion formed on the bottom surface, and the side wall surface. In addition, the NOx sensor element according to the above aspect (16), which has a tunnel-type electrode structure including a side electrode portion that connects the ceiling electrode portion and the bottom electrode portion.
According to the configuration of this aspect (17), the pump electrode of the main pump means has a tunnel-type electrode structure, so that the area efficiency of the electrode is improved, and interference with NOx measurement based on oxygen concentration change is prevented. Can be advantageously reduced.

(18) It has a pair of auxiliary pump electrodes formed inside and outside the second space, and oxygen contained in the atmosphere introduced from the first space is between the pair of auxiliary pump electrodes. The NOx sensor element according to any one of the above aspects (15) to (17), in which auxiliary pumping means for performing pumping processing based on the auxiliary pump voltage applied to is provided.

(19) The auxiliary pump electrode formed and arranged in the second space has a ceiling electrode portion formed on the ceiling surface of the second space, a bottom electrode portion formed on the bottom surface, and a side. The NOx sensor element according to the above aspect (18), which has a tunnel-type electrode structure, which is formed on a wall surface and includes a side electrode portion that connects the ceiling electrode portion and the bottom electrode portion.
According to such a configuration of the aspect (19), the area of the auxiliary pump electrode can be secured in an area sufficiently larger than the NOx measurement electrode, thereby effectively reducing the offset current of the NOx measurement output. It can be done.

(20) A clogging prevention space that opens outward is formed at the tip of the element, and the opening of the clogging prevention space constitutes the gas introduction port, and the clogging prevention space The NOx sensor element according to any one of the above aspects (15) to (19), wherein the first diffusion rate-limiting means is provided between the buffer space and the buffer space.

(21) The NOx sensor element according to any one of the above aspects (15) to (20), wherein the first and second diffusion rate-limiting means are each formed in a slit shape having a gap of 10 μm or less. .

(22) A porous ceramic layer is integrally formed on the first outer pump electrode formed outside the first cavity, and the first outer side is formed by the porous ceramic layer. The NOx sensor element according to any one of the above aspects (15) to (21), wherein the entire pump electrode is covered.

(23) A heater layer for electrically heating the element is laminated and integrated in the laminated structure of the plurality of solid electrolyte layers, and at least the first space and the heater are heated by the heater layer. The NOx sensor element according to any one of the above aspects (15) to (22), wherein the solid electrolyte layer portion defining the second space can be heated to a predetermined temperature.

(24) The heater layer includes a heater element having a heat generating portion, a current supply lead connected to the heat generating portion, and a resistance detection lead connected to a connection portion between the heat generating portion and the current supply lead. The lead resistance: R _L that is configured and detected using the resistance detection lead, and the overall heater resistance: Ra detected using the current supply lead:
R _H = Ra-2 × R _L
The NOx sensor element according to the above aspect (23), wherein the voltage applied to the heater element is controlled so that the resistance: _RH of the heat generating portion of the heater element is constant.
According to the configuration of the aspect (24), the heater temperature is controlled so that the resistance (R _H ) of the heater heat generating portion becomes constant, whereby the element temperature can be controlled more accurately. Produce.

  Hereinafter, in order to clarify the present invention more specifically, the configuration of the present invention will be described in detail with reference to typical examples shown in the drawings.

  First, FIGS. 1 to 4 show a schematic structure of a typical NOx sensor element according to the present invention, in which FIG. 1 is a longitudinal sectional explanatory view showing a laminated structure of such an element, FIG. 2 is a cross-sectional view of the element taken along the line II-II in FIG. 3 is an enlarged explanatory view taken along the line III-III in FIG. 2, and FIG. 4 is an explanatory view showing a planar structure of the heater element embedded in the laminated structure of the NOx sensor element shown in FIG.

  In these drawings, reference numeral 2 denotes a sensor element having an elongated plate-like body shape, and the sensor element 2 has a plurality of dense and airtight oxygen ion conductivity, as is apparent from FIG. The solid electrolyte layers 4 a, 4 b, 4 c, 4 d, 4 e, and 4 f are laminated to form a plate-like body having an integral structure. Each of the solid electrolyte layers 4a to 4f is formed using a known oxygen ion conductive solid electrolyte material such as zirconia porcelain, and the sensor element 2 having such an integral structure is similar to the conventional one. Thus, it can be easily obtained by firing and integrating the laminated structure of the unfired solid electrolyte layer.

  Further, in the sensor element 2 having such an integrated structure, a solid electrolyte layer 4a positioned at the top in the drawing and a solid electrolyte layer 4c positioned third from the top are interposed via a solid electrolyte layer 4b serving as a spacer layer. Thus, by stacking and integrating, the internal space located between the solid electrolyte layers 4a and 4c is, in other words, at a height corresponding to the thickness of the solid electrolyte layer 4b, as is apparent from FIG. For example, a space between the solid electrolyte layers 4a and 4c where the solid electrolyte layer 4b does not exist is formed so as to extend in the element longitudinal direction. In a form independent of such an internal space, the reference air introduction passage 6 serving as a reference gas existence space is defined as a space where the solid electrolyte layer 4c does not exist between the solid electrolyte layers 4b and 4d. The reference air introduction passage 6 is provided so as to extend in the longitudinal direction of the element 2 and is opened at the end portion on the base side of the sensor element 2 so as to communicate with the atmosphere, as in the conventional case. It has become.

  Further, in the sensor element 2, the internal space formed between the two solid electrolyte layers 4 a and 4 c is, as is apparent from FIGS. 1 and 2, buffer spaces 8 each having a rectangular planar shape. Since the first cavity 10 and the second cavity 12 are separately partitioned and are sequentially arranged so as to extend with a predetermined width in the element longitudinal direction. is there. Further, the clogging prevention cavities 14 that are opened outward and do not have the solid electrolyte layer 4b serving as the spacer layer so as to be located at the same level as the internal cavities are two solid electrolytes at the tip of the device. An opening of the clogging prevention space 14 is formed between the layers 4a and 4c, and serves as a gas inlet 16 for taking in a gas to be measured outside the element.

  The clogging prevention space 14 and the buffer space 8 are partitioned by a first partition wall 18 constituted by the solid electrolyte layer 4b, and the first partition wall 18 has an upper portion and a lower portion. As shown in FIG. 2, the upper and lower solid electrolyte layers 4a and 4c have a first diffusion rate-determining method in a slit shape extending in the element width direction having substantially the same width as the buffer space 8, as shown in FIG. A first diffusion rate-determining passage 20 serving as a means is formed, and the external space to be measured introduced from the gas inlet 16 through the clogging prevention space 14 through the upper and lower first diffusion rate-limiting passages 20 and 20. Gas can be introduced into the buffer space 8 under a predetermined diffusion resistance.

  Further, even in the second partition wall 22 that partitions the buffer space 8 and the first space 10, it is provided by the solid electrolyte layer 4 b and the first partition wall 18 described above. Similarly, in the upper and lower portions, the second diffusion rate controlling passages 24 and 24 as the second diffusion rate controlling means are slits extending in the element width direction between the solid electrolyte layers 4a and 4c outside the element and inside the element, respectively. An atmosphere (gas to be measured) in the buffer space 8 is formed from the buffer space 8 into the first space 10 through the second diffusion rate-determining passage 24 under a predetermined diffusion resistance. It can be introduced. Then, in the pumping operation of the main pump means which is an electrochemical pump cell composed of the solid electrolyte layer 4a and the inner pump electrode 26 and the outer pump electrode 28 formed on the inner and outer surfaces, the first empty layer is formed. The oxygen in the atmosphere in the space 10 is pumped into the external space, or the oxygen in the external space is pumped into the first space 10, and the oxygen concentration (partial pressure) in the atmosphere in the first space 10 is As in the prior art, the concentration is controlled to a predetermined level.

  In such an element structure, the buffer space 8 is provided, and the slit-shaped first and second diffusion-controlling passages 20 are provided in the first partition wall 18 and the second partition wall 22, respectively. , 24 can provide the following benefits. That is, normally, oxygen rushes into the internal space of the sensor element 2 through the gas inlet 16 due to the pulsation of the exhaust pressure in the external space. In the illustrated structure, oxygen from the external space is introduced. Does not directly enter the internal space (processing space), but enters the buffer space 8 in the preceding stage through the first diffusion-controlled passage 20 and further passes through the second diffusion-controlled passage 24 to the internal space. It is introduced into the first void 10 that is the place. Therefore, a sudden change in the oxygen concentration due to the pulsation of the exhaust pressure is canceled out by the buffer space 8 and the first and second diffusion-controlled passages 20 and 24 provided before and after the buffer space 8 and the internal space (the first space). The influence of the pulsation of the exhaust pressure on the place 10) is almost negligible. As a result, the correlation between the oxygen pumping amount at the pump means in the processing space and the oxygen concentration in the gas to be measured can be improved, and the measurement accuracy can be improved. It can also be used as a sensor for obtaining the air-fuel ratio. In order to advantageously exhibit such an effect, the first and second diffusion rate limiting passages 20 and 24 provided in the first and second partition walls 20 and 24 respectively have a gap of 10 μm or less. It is desirable to be provided in a slit shape.

  In addition, the clogging prevention space 14 opened outward at the tip of the element is buffered by particulate matter (soot, oil combustion product, etc.) generated in the gas to be measured introduced through the gas introduction port 16 in the external space. It is provided so as to avoid clogging in the vicinity of the entrance of the void 8, which makes it possible to measure the NOx component with higher accuracy and to maintain the high accuracy state over a long period of time. Such an arrangement of the anti-clogging space 14 can be advantageously employed because it exhibits the feature that it can be maintained.

By the way, in the main pump means arranged with respect to the first cavity 10, the inner pump electrode 26 and the outer pump electrode 28 are generally porous cermet electrodes, for example, a metal such as Pt and ZrO ₂ or the like. The inner pump electrode 26 disposed in the first space 10 that is in contact with the gas to be measured does not cause a change in the NOx component in the gas to be measured. In other words, it is necessary to use a material having reduced reduction ability or decomposition ability for NOx components such as NO and NO ₂ or having no such reduction / decomposition ability. For example, a perovskite structure such as La ₃ CuO ₄ is used. Cermet of a compound having a low catalytic activity such as Au and a ceramic material, or a metal having a low catalytic activity such as Au, a Pt group metal and a ceramic material And thus it is composed of cermet with. Here, a porous protective layer 29 made of alumina or the like is provided so as to cover the outer pump electrode 28, so that oil components and the like contained in the gas to be measured in the outer space do not adhere to the outer pump electrode 28. Thus, the outer pump electrode 28 can be protected.

  Further, as shown in FIG. 1, the inner pump electrode 26 constituting the main pump means straddles the upper and lower solid electrolyte layers 4a and 4c that define the first cavity 10 and the solid electrolyte layer 4b that provides the side walls. Is formed. Specifically, a ceiling electrode portion 26a is formed on the lower surface of the solid electrolyte layer 4a that provides the ceiling surface of the first cavity 10, and a bottom electrode portion is formed on the upper surface of the solid electrolyte layer 4c that provides the bottom surface. 26b is formed, and the side electrode portion 26c is on the side of the solid electrolyte layer 4b constituting both side walls of the first space 10 so as to connect the ceiling electrode portion 26a and the bottom electrode portion 26b. In the tunnel-type electrode structure formed on the wall surface (inner surface) and having a tunnel shape at the portion where the side electrode portion 26c is provided, it is provided.

  Furthermore, in this exemplary sensor element 2, a third partition wall 30 that partitions the first space 10 and the second space 12 is provided by the solid electrolyte layer 4b and is shown in FIG. As described above, in the upper and lower portions, like the first and second partition walls 18 and 22, the second space 12 extending in the element width direction between the solid electrolyte layer 4a and the solid electrolyte layer 4c. In the slit shape having a length substantially equal to the width, a third diffusion rate-limiting passage 32 is formed as third diffusion rate-limiting means for communicating the first space 10 and the second space 12. An atmosphere in which the oxygen concentration (partial pressure) in the first cavity 10 is controlled through the third diffusion-controlling passage 32 is guided into the second cavity 12 under a predetermined diffusion resistance. It has become.

An auxiliary pump electrode 34 and a measurement electrode 36 are provided in the second space 12. The auxiliary pump electrode 34 and an appropriate external electrode, for example, the outer pump electrode 28, are solidified. The oxygen concentration (partial pressure) in the atmosphere in the second space 12 can be controlled to a predetermined concentration by combining the electrolyte layer 4a and configuring as an auxiliary electrochemical pump cell. . In addition, the auxiliary pump electrode 34 is provided in the second cavity 12 in a tunnel electrode structure similar to the inner pump electrode 26 provided in the first cavity 10. . That is, the ceiling electrode portion 34a is formed on the solid electrolyte layer 4a that provides the ceiling surface of the second cavity 12, and the bottom electrode is provided on the solid electrolyte layer 4c that provides the bottom surface of the second cavity 12. 34b is formed, and side electrode portions 34c that connect the ceiling electrode portion 34a and the bottom electrode portion 34b are formed on both side surfaces of the solid electrolyte layer 4b that provide the side wall of the second space 12, respectively. Therefore, it is a tunnel structure. The auxiliary pump electrode 34 is made of a material that has weakened or has no reduction / decomposition ability for the NOx component in the gas to be measured, like the inner pump electrode 26 in the main pump means. For example, it is formed of a porous cermet electrode of Pt and ZrO ₂ containing 1% of Au.

  Furthermore, the measurement electrode 36 disposed in the second space 12 is brought into contact with an atmosphere in which the oxygen concentration (partial pressure) in the second space 12 is controlled, and thus the measurement electrode 36 is placed in the atmosphere. It is necessary to include a component that causes reduction or decomposition of the contained NOx component. Here, an electrode metal material that can reduce or decompose the NOx component in the gas to be measured is used. It is formed in the form of a porous electrode made of cermet with a ceramic material. In addition, as an electrode metal material for providing such a cermet constituting the measurement electrode 38, a noble metal is advantageously used, and an alloy of Pt (platinum) or Pt and Rh (rhodium) is particularly advantageous among noble metals. As the alloy ratio of Pt and Rh, a ratio that satisfies the ratio of Pt: Rh = 100 to 40% by weight: 0 to 60% by weight is preferably employed. Further, when a noble metal is used as the electrode metal material, the ratio (vol%) between such noble metal and ceramic material is a value within the range of noble metal / ceramic material = 65/35 to 40/60. It will be advantageously employed.

In addition, as a ceramic material which is another component that provides the cermet constituting the measurement electrode 36, a ZrO ₂ material is advantageously used for strong bonding with the solid electrolyte layer 4c. , Y ₂ O containing ₃ at a rate of 5.5~8.5mol% Y ₂ O ₃ stabilized ZrO ₂ material is particularly advantageously used. By using such ZrO ₂ material stabilized with a specific amount of Y ₂ O ₃ , when heating and cooling are repeated by repeated measurement or the like, the electrode skeleton is caused by T / M transformation of ZrO _2. As a result, the durability of the sensor element 2 can be advantageously improved by effectively suppressing or preventing the deterioration of the function as an electrode and the occurrence of problems such as electrode peeling and cracking. In addition, when the content of Y ₂ O ₃ is less than 4.5 mol%, such a problem is likely to be caused, and more than 8.5 mol% is contained, and ZrO ₂ is more stable. Even if it is made to form, the strength of ZrO ₂ decreases, and the same problem as described above is caused.

Incidentally, an inert component such as a metal volatilized from the auxiliary pump electrode 34 similarly disposed in the second space 12 adheres to the measurement electrode 36 disposed in the second space 12. 1 and 3 so that the catalytic activity (NOx decomposition / reduction ability) of the measuring electrode 36 can be effectively maintained, as shown in FIGS. The electrode protective layer 38 made of a porous ceramic layer formed in this manner is formed with a predetermined thickness using a ceramic material such as Al ₂ O ₃ .

  In this case, the electrode protective layer 38 is formed so as to have a flat upper surface (surface) at least on the measurement electrode 36, and as shown in FIG. In a cross-sectional form having a substantially trapezoidal shape having a flat upper base portion 38a extending to the side and height changing portions 38b and 38b whose height gradually decreases from both ends of the upper base portion 38a toward the sides of the measuring electrode 36, respectively. As a result, the generation of cracks and peeling of the electrode protective layer 38 can be effectively suppressed or prevented. Although the reason has not yet been fully elucidated, a sufficient thickness can be ensured at both ends of the measurement electrode 36, possibly by making the electrode protective layer 38 into a trapezoidal cross-sectional shape. As a result, the strength of the electrode protective layer 38 can be effectively increased, so that even if stress is generated due to repeated oxidation / reduction of the electrode metal in the cermet that provides the measurement electrode 36, sufficient resistance is exerted. It is considered that cracks generated from the vicinity of both ends of the measuring electrode 36 can be effectively prevented.

In the electrode protective layer 38, the width (W ₁ ) of the upper base 38a in the trapezoidal cross-sectional shape is larger than the width (W ₂ ) of the measurement electrode 36 (W ₁ > W ₂ ), The measurement electrode 36 is desirably protruded laterally from both ends, and the electrode protective layer 38 is formed so as to satisfy the following formula: W ₁ ≧ 1.1 × W _2. It is more desirable. Here, the width (W ₂ ) of the measurement electrode 36 is the width at the interface between the measurement electrode 36 and the solid electrolyte layer 4c (the bottom surface of the measurement electrode 36). Thus, as shown in FIG. 1 and FIG. 3, the electrode protective layer 38 is further extended laterally from the left and right end portions of the measurement electrode 36 so as to be protruded. Prevention of cracking and peeling of the protective layer 38 can be achieved more advantageously.

In particular, in the present invention, the electrode protective layer 38 is formed on the thickness of the measurement electrode 36 and on the side positions P ₁ and P ₂ that are 80 μm apart from the left and right ends of the measurement electrode 36, respectively. to have the sum of the thickness of the electrode protective layer 38 located the (t ₂₎ to a thickness which is a value obtained by multiplying _{0.9~1.0 (t 1 = 0.9t 2 ~1.0t} 2) However, it is more desirable, whereby the object of the present invention can be achieved more advantageously.

  Here, at least the upper surface of the electrode protective layer 38 located on the measurement electrode 36 is formed by baking the electrode protective layer 38 formed by screen printing, so that it extends completely in the horizontal direction. In the present specification, the presence of unevenness up to 8 μm is tolerated in the present specification because it is not provided as a flat surface but is provided on a flat surface where a certain degree of unevenness exists. Also in the trapezoidal cross-sectional shape of the layer 38, the upper bottom portion 38a whose height (thickness) changes at 8 μm or less due to such unevenness is treated as being substantially flat. In addition, the height change portions 38b and 38b at both ends of the electrode protection layer 38, in other words, the height change portion linearly changes in height even in the trapezoidal hypotenuse, As shown in FIG. 3, it may be configured as a height changing portion whose height changes in a curve.

  In the exemplary sensor element 2, the measurement electrode 36 and the electrode protective layer 38 constituting the NOx measurement electrode part structure are integrally formed on the solid electrolyte layer 4 c in the second space 12. Furthermore, without contacting the side wall formed by the solid electrolyte layer 4b that defines the second space 12, the side wall surface is in other words through a predetermined space between the side wall and the side wall. In a form spaced from the As a result, the occurrence of a problem based on the difference in thermal shrinkage caused when the electrode protective layer 38 and the measurement electrode 36 are in contact with the side wall surface constituted by the solid electrolyte layer 4b, specifically, In this case, the problem that the electrode protective layer 38 and the like peel off from the side wall surface of the solid electrolyte layer 4b and adversely affect the NOx measurement electrode part structure can be effectively avoided.

The measurement electrode 36 and the electrode protective layer 38 constituting the NOx measurement electrode part structure are formed in an appropriate thickness and size according to the size of the second space 12 where they are provided. In particular, in the present invention, the thickness of the measurement electrode 36 is preferably 15 to 35 μm, and the electrode protective layer 38 is preferably 20 to 45 μm in thickness. In order to effectively achieve the desired effect of the invention, it is desirable that the sum (t ₂ ) of the thickness of the measurement electrode 36 and the thickness of the electrode protective layer 38 be 70 μm or less. However, if the thickness of the measuring electrode 36 is too thin, the function as an electrode is reduced, and if the thickness is too thick, cracks or peeling may occur when the vehicle is used for a long time. It is because it becomes easy to generate | occur | produce. Further, if the thickness of the electrode protective layer 38 becomes too thin, there is a problem that the function as the protective layer is lowered. On the other hand, if the thickness becomes too thick, the NOx measurement electrode part structure is integrally formed during firing. Problems such as cracks occurring in the solid electrolyte layer (4c) are caused.

  Further, in such a NOx measurement electrode portion structure, the longitudinal direction of the element as shown in FIG. 1 and FIG. 3, in other words, the main diffusion direction of the gas to be measured or the atmosphere in the internal space (the horizontal direction in the figure). The electrode width of the measuring electrode 36 is generally 0.2 to 0.8 mm, and the electrode protective layer 38 is added to the length (width) of the measuring electrode 36 by 200 to 600 μm. It is desirable to be formed in size, so that the object of the present invention can be better achieved.

  In the exemplary sensor element 2, the reference electrode 39 is arranged in such a form that it can be brought into contact with the reference air in the reference air introduction passage 6 on the opposite side of the solid electrolyte layer 4 c from the second space 12. The oxygen concentration (partial pressure) in the atmosphere in the first space 10 and the second space 12 can be measured using the reference electrode 39. In particular, if the measurement electrode 36, the solid electrolyte layers 4c and 4d, and the reference electrode 39 are combined to constitute an oxygen partial pressure detection means as an electrochemical sensor cell, the measurement electrode 36 is included in the atmosphere around the measurement electrode 36. The electromotive force according to the difference between the amount of oxygen generated by the reduction or decomposition of the NOx component generated and the amount of oxygen contained in the reference air can be detected, whereby the concentration of the NOx component in the gas to be measured Is also possible. Here, the reference electrode 39 is provided on the solid electrolyte layer 4d as a seal layer, and further, a porous alumina layer 40 for introducing air is provided so as to cover it, and a reference air introduction passage is provided. The reference air in 6 can be brought into contact with the reference electrode 39 through the porous alumina layer 40.

  Further, in the sensor element 2, as shown in FIG. 1, a plurality of ceramic layers, that is, solids are provided on the side of the solid electrolyte layer 4c opposite to the side where the internal spaces (8, 10, 12) are disposed. The electrolyte layers 4d to 4f are laminated and integrated, and in the form sandwiched from above and below by two adjacent solid electrolyte layers 4d and 4e, there is provided a heater layer 42 that generates heat by external power feeding. Yes. The heater layer 42 is provided to heat them to a predetermined temperature in order to increase the conductivity of oxygen ions in the solid electrolyte layers 4a to 4f constituting the sensor element. It is arranged in a form sandwiched from above and below by an electrical insulating layer such as alumina for obtaining electrical insulation from the solid electrolyte layers 4d and 4e. Further, such a heater layer 42 is provided on the sensor element base side. The pressure diffusion hole 45 penetrating the solid electrolyte layer 4d is communicated with the reference air introduction passage 6 so that the internal pressure rise in the heater layer 42 can be alleviated. The heater element 44 of the heater layer 42 is taken out to the element surface through a through hole 46 that is provided through the solid electrolyte layers 4e and 4f and is insulated from the surroundings. Further, what is the solid electrolyte layer 4f? The connector pad 47 formed by insulation is made conductive.

And the heater element 44 in this heater layer 42 is comprised so that the solid electrolyte layer 4a-4c part which divides at least the 1st space 10 and the 2nd space 12 can be heated to predetermined temperature. Here, as shown in FIG. 4, a heating part 44a for heating the vicinity of the arrangement site of the first and second cavities 10, 12 is connected to both ends of the heating part 44a. Connected to current supply leads 44b and 44b for supplying a predetermined heater current to the heat generating portion 44a and a connection portion between one of the heat generating portions 44a and the current supply leads 44b and 44b (here, upstream one in the energizing direction). The resistance detection lead 44c. Then, from the heater lead resistance _RL detected using one current supply lead 44b and the resistance detection lead: R _L and the overall heater resistance Ra obtained using the two light bulb supply leads 44b, 44b, the following formula:
R _{H =} Ra-2 × R _L
Based on the above, the resistance: _RH of the heater heat generating portion 44a is calculated, and the voltage applied to the heater element 44 is controlled by a control device (not shown) so that the heater heat generating portion resistance: _RH becomes constant. As a result, the element temperature can be controlled more accurately.

  In such a NOx sensor element 2, an electrochemical pump cell, that is, a main pump cell 50 is constituted by the solid electrolyte layer 4 a and the inner and outer pump electrodes 26 and 28. In order to detect the oxygen concentration (partial pressure), an electrochemical sensor cell, that is, an oxygen partial pressure detecting cell 52 for controlling the main pump cell is formed from the solid electrolyte layers 4a to 4d, the inner pump electrode 26 and the reference electrode 39. It is configured. Here, reference numeral 54 denotes a variable power source for driving the main pump cell 50.

  In order to control the oxygen partial pressure in the atmosphere in the second space 12, an electrochemical pump cell comprising the solid electrolyte layer 4a, the outer pump electrode 28, and the auxiliary pump electrode 34, that is, the auxiliary pump cell 56 is provided. An electrochemical sensor cell comprising a solid electrolyte layer 4a, 4b, 4c, 4d, an auxiliary pump electrode 34 and a reference electrode 39 to detect the partial pressure of oxygen in the second cavity 12; That is, an auxiliary pump cell control oxygen partial pressure detection cell 58 is configured. The auxiliary pump cell 56 is adapted to be pumped by a variable power source 60 that is voltage-controlled by the auxiliary pump cell control oxygen partial pressure detection cell 58, and the pump current value Ip1 is the main pump cell. The control oxygen partial pressure detection cell 52 is used to control the electromotive force: V0.

  Further, in order to pump out oxygen generated by decomposition of nitrogen oxide (NOx) in the atmosphere around the measuring electrode 36 and detect the amount of the generated oxygen, the solid electrolytes 4a, 4b, 4c, the outer pump electrode 28, the measuring electrode 36, an electrochemical pump cell, that is, a measurement pump cell 62, is constructed. On the other hand, a measurement pump cell control oxygen partial pressure detection cell 64 for detecting the oxygen partial pressure in the atmosphere around the measurement electrode 36 is formed. However, the solid electrolyte layers 4a, 4b, 4c, and 4d, the measurement electrode 36, and the reference electrode 39 constitute an electrochemical sensor cell. The measuring pump cell 62 is pumped by the variable power source 64 whose voltage is controlled based on the electromotive force: V2 detected by the measuring pump cell controlling oxygen partial pressure detecting cell 64, and the gas to be measured A pump current value Ip2 corresponding to the concentration of nitrogen oxide in the inside is obtained.

  The solid electrolytes 4a, 4b, 4c, and 4d, the outer pump electrode 28, and the reference electrode 39 also constitute an electrochemical sensor cell 68. The electromotive force obtained by the sensor cell 68: Vref causes the outside of the sensor. The oxygen partial pressure (concentration) in the gas to be measured can be detected.

When detecting the nitrogen oxide (NOx) concentration in the gas to be measured using the nitrogen oxide sensor having such a configuration, first, the external gas to be measured is clogged at the tip of the sensor element 2. After being introduced into the buffer space 8 from the prevention space 14 through the slit-shaped first diffusion rate limiting passages 20 provided above and below the first partition wall 18, further provided above and below the second partition wall 22. Then, it is introduced into the first space 10 through the slit-shaped second diffusion rate-determining passage 24, and is variable so that the electromotive force V0 in the oxygen partial pressure detection cell 52 for main pump cell control is constant. The pump current: Ip0 in the main pump cell 50 is controlled by controlling the voltage of the power supply 54. Here, the oxygen partial pressure in the atmosphere in the first space 10 is controlled to be a predetermined value, for example, about 10 ⁻⁷ atm.

  The atmosphere led from the first space 10 into the second space 12 through the slit-shaped third diffusion rate controlling passages 32 above and below the third partition wall 30 is the oxygen partial pressure for controlling the auxiliary pump cell. Electromotive force detected by the detection cell 58: The auxiliary pump cell 56 performs the oxygen pumping operation by the power supply from the variable power source 60 that is voltage-controlled based on V1, and oxygen in the atmosphere in the second space 12 The partial pressure is controlled to a low oxygen partial pressure value that does not substantially affect the measurement of nitrogen oxides. Further, the pumping current Ip1 in the auxiliary pump cell 56 is input as a control signal to the main pump cell control oxygen partial pressure detection cell 52, and the electromotive force V0 is controlled, so that the third diffusion rate-limiting path is obtained. The gradient of the oxygen partial pressure in the atmosphere in the second space 12 extending from 32 to the auxiliary pump electrode 34 is controlled so as to be always constant.

  Further, in the second space 12, the atmosphere in which the oxygen partial pressure is controlled reaches the measurement electrode 36 through the electrode protective layer 38 under a predetermined diffusion resistance. Nitrogen oxide in the atmosphere is reduced or decomposed around the measurement electrode 36 to generate oxygen. Then, the generated oxygen is pumped by the measurement pump cell 62. At this time, the variable power source is set so that the electromotive force: V2 in the measurement pump cell control oxygen partial pressure detection cell 64 is constant. 66 voltage is controlled. Here, since the amount of oxygen generated around the measurement electrode 36 is proportional to the concentration of nitrogen oxide in the gas to be measured, the pump current Ip2 in the measurement pump cell 62 is used. Thus, the concentration of nitrogen oxide (NOx) in the target gas to be measured is calculated.

  By the way, the sensor element 2 having such a structure can basically be manufactured by appropriately selecting a method conventionally employed for manufacturing an element having a laminated structure. The green sheets for providing the solid electrolyte layers 4a, 4b, 4c, etc., on which the predetermined electrodes are provided when the unfired solid electrolyte tapes (green sheets) for providing the electrolyte layers 4a to 4f are sequentially laminated. An electrode paste that forms a cermet electrode by firing is printed on a predetermined portion, and the green sheet that gives the solid electrolyte layers 4b and 4c has a clogging prevention space 14, a buffer space 8, and a first space. The space for forming the space 10 and the second space 12 and the space for forming the reference air introduction passage 6 are formed by pressing or the like, and further, the solid electrolyte layer 4d, e and 4f are punched for the pressure diffusion hole 45 and the through hole 46, and the heater layer 42 is formed between the green sheets for providing the solid electrolyte layers 4d and 4e. The first, second and third partition walls 18, 22, 30 are provided in order to integrate the layers and form the first, second and third diffusion-determining channels 20, 24, 32 in a slit shape. Adjustment is performed by forming a layer of a material that disappears by firing such as theobromine to a predetermined thickness on the upper and lower surfaces of the green sheet (4b) and the green sheet surface that gives the solid electrolyte layers 4a and 4c facing them. Thus, the sensor element 2 having the target element structure can be obtained.

  When manufacturing such a sensor element 2, such a NOx measurement electrode part structure is configured to advantageously form the NOx measurement electrode part structure (36, 38) in the second space 12. The electrode protective layer 38 is advantageously formed on the measurement electrode 36 in a form having a substantially trapezoidal cross-sectional shape by a screen printing technique using a metal mask. That is, the screen printing technique using this metal mask is well-known and, as schematically shown in FIG. 5, a predetermined amount is applied on the green sheet 70 that gives the solid electrolyte layer (4c). In this pattern, a measurement electrode forming layer 72 made of an electrode material that provides the measurement electrode 36 is provided, and then a pattern corresponding to the shape of the target electrode protective layer 38 is punched out. A metal paste (metal plate) 74 is overlaid, and then a printing paste for providing the electrode protection layer 38 is screen-printed using an appropriate squeegee such as a plastic squeegee 76 according to the same operation as before, and The electrode protective layer forming layer 78 is screen-printed on the measurement electrode forming layer 72, and then integrally fired together with the green sheet 70. And a is the electrode protective layer 38 having a sectional form of a substantially trapezoidal shape such as shown in FIG. 3 may be advantageously formed.

  In such metal mask printing, as the metal mask 74, a metal plate (foil) such as SUS having a thickness of about 50 to 100 μm is generally used, and even a squeegee such as a plastic squeegee 76 is used. In addition, a hard squeegee such as plastic or metal is advantageously used. Further, as a printing paste for providing the electrode protective layer 38, a comparison is made in order to more effectively realize the target trapezoidal cross-sectional shape of the electrode protective layer 38. In particular, a material having a high viscosity, for example, a material having a viscosity of about 100 to 300 Pa · s is preferably used.

  Prior to such metal mask printing, the measurement electrode forming layer 72 formed on the green sheet 70 can be formed according to various known techniques. Generally, this is a common screen printing technique. Will be formed. At that time, the measurement electrode forming layer 72 can be formed not only by a normal screen printing technique using a mesh screen but also by adopting a metal mask printing method as described above.

  In addition, although typical embodiment of this invention has been described in detail, it cannot be overemphasized that this invention is not limited to the thing of the form of such an illustration. It goes without saying that the present invention can be implemented in embodiments with various changes, modifications, improvements and the like based on the knowledge of those skilled in the art. It should be understood that all belong to the scope of the present invention without departing from the spirit of the invention.

  In addition, representative examples of the present invention will be shown below to further clarify the features of the present invention. However, the present invention is not limited by the description of such examples. It goes without saying that it is not something that you receive.

<Example 1>
First, a test piece provided with a NOx measurement electrode part structure was produced as shown in FIGS. That is, as shown in FIG. 6, an electrode paste that gives a measurement electrode (36) to a ZrO ₂ green sheet 80 having a thickness of about 250 μm is 0.5 mm by a screen printing method using a normal mesh screen. A measurement electrode forming layer 82 having a size of × 1.25 mm was printed and dried at a thickness of about 25 μm. The electrode paste was prepared by appropriately adding an organic binder, a solvent and a plasticizer to (Rh / Pt = 50/50 wt% powder) / (ZrO ₂ powder) = 60/40 vol%. Next, the Al ₂ O ₃ paste is used to print the insulating layer 84 under the electrode lead on the green sheet 80 to a thickness of about 13 μm by a screen printing method using a normal mesh screen, and then dry. The electrode lead 86 was printed on the insulating layer 84 having the electrode lead to a thickness of about 13 μm by a screen printing method using a normal mesh screen and dried. Here, the paste used to form the electrode lead 86 is made of ZrO ₂ which is equivalent to the green sheet 80 or has a higher sinterability than the green sheet 80 so that the electrode lead 86 becomes dense after firing. In addition to powder / ZrO ₂ powder = 60/40 vol%, an organic binder, a solvent and a plasticizer were appropriately added. On the electrode lead 86, an insulating layer 88 on the electrode lead is printed to a thickness of about 13 μm by a screen printing method using a normal mesh screen with an Al ₂ O ₃ paste that becomes a dense film after firing. , Dried.

After that, on the measurement electrode forming portion 82, an Al ₂ O ₃ paste that gives a porous layer after firing is used, and a size of 0.9 mm × 1.75 mm is obtained by a screen printing method using a metal mask. The electrode protective layer forming portion 90 was printed at a thickness of about 35 μm and dried to obtain a laminated printed material having a cross-sectional structure as shown in FIG.

  Next, a green sheet 92 provided with a notch corresponding to an internal space for providing the solid electrolyte layer (4b) is overlaid on the obtained multilayer printed material, and after being cut into element dimensions, in the atmosphere, The target test piece A was obtained by firing at a temperature of 1300 to 1400 ° C. As a result of cross-sectional observation of the obtained test piece A by SEM, the thicknesses of the measurement electrode and the electrode protective layer formed by firing were 18 to 24 μm and 24 to 31 μm, respectively. The total thickness of the electrode protective layer located at 46 was 46 to 50 μm. Further, the dimensions of the measurement electrode and the electrode protective layer after firing are the dimensions in the horizontal direction of FIG. 8, the measurement electrode has an electrode width of about 400 μm, and the electrode protective layer has a dimension at its upper bottom. Was about 560 μm, and the size of the lower bottom portion was about 720 μm.

On the other hand, for comparison, a laminated printed material was produced in the same manner as described above except that an Al ₂ O ₃ paste for forming an electrode protective layer was printed by a screen printing method using a normal mesh screen. Then, after laminating the green sheets 92 for forming the internal voids, a similar baking operation was performed to obtain a test piece B as a comparative example. In addition, the thickness of the measurement electrode and the electrode protective layer in the test piece B obtained by firing is 18 to 24 μm and 22 to 32 μm, respectively, and the total thickness of the measurement electrode and the electrode protective layer is 44. It was ˜51 μm. Moreover, the length of the flat part of the upper surface of the electrode protective layer in this test piece B was about 300 μm.

  And about the test pieces A and B obtained in this way, cross-section observation was carried out by SEM, and the SEM cross-section photograph corresponding to each Fig.8 (a) was shown in FIG.9 and FIG.10, respectively. 9 and 10, the test piece A shows a trapezoidal cross section with a long flat top bottom (FIG. 9), whereas the test piece A In this case, it is recognized that the upper surface of the electrode protective layer is curved in a dome shape and does not have an effective trapezoidal shape (FIG. 10).

  Next, each of the test pieces A and B thus obtained was attached to an exhaust pipe of an experimental engine bench (3.5 L / V6 gasoline engine) and subjected to durability evaluation by exposing it to exhaust gas. The operation pattern of the gasoline engine was a life cycle pattern, and the gas temperature was changed within a range of 150 ° C to 600 ° C. And the test piece was taken out every 20 hours, and it was observed with the microscope whether the electrode protective layer was cracked or not. And the time when generation | occurrence | production of the crack was recognized with the microscope was made into failure time.

  As a result of the durability test, in the test piece B of the comparative example, cracks started to occur from 43 hours, and it was confirmed that cracks occurred in almost half of the test pieces in 100 hours. On the other hand, in the test piece A according to the present invention, even when the durability reached 900 hours, no failure (crack) occurred, and from the point where the total number of test pieces passed, As a result, the life of the test piece A was estimated. As is apparent from the graph of FIG. 11 showing the result, when compared with the time to failure rate: 0.1%, the test piece A according to the present invention has 500 hours and the test piece B according to the comparative example has 33 hours. As a result, it was confirmed that the test piece A had a durability performance 15 times or more that of the test piece B.

<Example 2>
In the same manner as in Example 1, except that the ZrO ₂ material used for preparing the electrode paste for forming the measurement electrode was replaced with four ZrO ₂ materials having different Y ₂ O ₃ contents. Test pieces (A) to (D) were prepared. Note that the test piece (A) uses a partially stabilized ZrO ₂ material added with 5 mol% of Y ₂ O ₃ as a ceramic material for electrode paste formation. Similarly, the test piece (B) is Y ₂ A partially stabilized ZrO ₂ material to which 6 mol% of O ₃ is added is used, and a test piece (C) is a completely stabilized ZrO ₂ material to which 8 mol% of Y ₂ O ₃ is added. Furthermore, the test piece (d) uses a partially stabilized ZrO ₂ material added with 4 mol% of Y ₂ O ₃ , and the electrode protective layer is formed in the same manner as the test piece (B) in Example 1. This relates to a comparative example formed by screen printing using a mesh screen. Among the test pieces obtained, test pieces (a) to (c) had the same trapezoidal cross-sectional shape as test piece A of Example 1, but test pieces (d) Has a dome-shaped cross-sectional shape similar to that of the test piece B.

  And, for each of the obtained test pieces (I) to (D), a durability test similar to that in Example 1 is performed, and whether or not a crack occurs in the electrode protective layer in each test piece. Observed with a microscope. In this durability test, the gas temperature was changed between 400 ° C and 800 ° C.

  As a result of the durability test, it was confirmed that the test piece (d) according to the comparative example started to crack after 43 hours and almost half of the cracks occurred after 100 hours. On the other hand, none of the test pieces (A) to (C) according to the present invention has a failure (occurrence of cracks) until 2000 hours of endurance, and since all of them passed, one of them failed. The lifetime was estimated with a virtual line, and the result is shown in FIG. As is apparent from FIG. 12, when the failure rate is compared to the time to reach 0.1%, the test pieces (A) to (C) according to the present invention are 1000 hours, and the test pieces (D ) Is 33 hours, it is confirmed that the test pieces (A) to (C) according to the present invention have 30 times or more durability performance compared to the test piece (d) of the comparative example. It was done.

<Example 3>
Various types of NOx sensor elements having the structure shown in FIG. 1 were prepared using the four types of electrode pastes for forming measurement electrodes prepared in Example 2. In order to form the solid electrolyte layers 4a to 4f, ZrO ₂ sheets (green sheets) having a thickness after firing of 0.2 ± 0.05 mm were used, and six of them were laminated in the same manner as in the past. On the other hand, the NOx measurement electrode part structure comprising the measurement electrode 36 and the electrode protective layer 38 formed in the second space 12 is the same as the test pieces (a) to (d) of the second embodiment. NOx sensor elements (A) to (D) corresponding to the respective formed NOx sensor elements were obtained. Therefore, in the NOx sensor elements (a) to (c), the electrode protective layer 38 is formed by a screen printing method using a metal mask, and the NOx sensor element (d) The electrode protective layer 38 is formed by a screen printing method using a mesh screen.

  Next, using the obtained NOx sensor elements (a) to (d), metal parts are assembled in the same manner as in the past, and each NOx sensor is completed, and this is used as a sample sensor. It was evaluated in a test.

  Specifically, sample sensors (each level N = 5) corresponding to the respective NOx sensor elements (A) to (D) are attached to the exhaust pipe of an experimental engine bench (3.5 L / V6 gasoline engine), Operation pattern: Life cycle pattern, gas temperature: Exposed to 400 to 800 ° C. exhaust gas, sample sensor was taken out every 100 hours, and NOx sensitivity of each sample sensor was measured with a model gas evaluation device. Then, the change rate was calculated by repeating the sensitivity after each endurance time with the initial sensitivity before endurance, and the result is shown in FIG.

  As is apparent from the durability test results of FIG. 13, in the case where the NOx sensor element (d) according to the comparative example is used, the sensitivity gradually increases from the start of durability to 700 hours, but after 700 hours, the sensitivity is increased. Turned to decrease, and after 1200 hours of durability, the sensitivity decreased to -25%. On the other hand, in the case of using the NOx sensor elements (a) to (c) according to the present invention, the sensitivity change rate is maintained within ± 3% until the endurance of 2100 hours at any level. A clear advantage over the example NOx sensor element (2).

<Example 4>
In the same manner as in Example 2, the electrode protective layer forming portions 90 were respectively formed by screen printing using a mesh screen using four types of electrode pastes for forming measurement electrodes, and corresponding four types of electrode pastes were formed. Test pieces (1) to (4) were produced. The test piece (1) was obtained using a partially stabilized ZrO ₂ material in which 5 mol% of Y ₂ O ₃ was added to the electrode paste constituent ceramic material. Similarly, the test piece (2) was , Obtained by using a partially stabilized ZrO ₂ material to which 6 mol% of Y ₂ O ₃ was added, and the test piece (3) was made of a completely stabilized ZrO ₂ material to which 8 mol% of Y ₂ O ₃ was added. Furthermore, the test piece (4) was obtained using a partially stabilized ZrO ₂ material to which 4 mol% of Y ₂ O ₃ was added.

  And about the obtained 4 types of test pieces (1)-(4), the endurance test was done like Example 2, respectively, and the result was shown in FIG. As is clear from the results of FIG. 14, in the test piece (4) according to the comparative example, cracks started to be generated from 43 hours, and it was confirmed that approximately half of the cracks were generated in 100 hours. On the other hand, in the test pieces (1) to (3) according to the present invention, no cracks were generated until the endurance of 1500 hours. In the case of a failure, the lifetime is estimated by a virtual line. Then, when the time to failure rate reaches 0.1%, the test pieces (1) to (3) according to the present invention are 950 hours, and the test piece (4) according to the comparative example is 33 hours. Thus, it was confirmed that the test piece according to the present invention had a durability performance of 28 times or more that of the test piece of the comparative example.

<Example 5>
The NOx sensor elements (1) to (4) are manufactured in the same manner as in Example 4 except that the electrode protection layer is formed, and the NOx sensor elements (1) to (4) are manufactured and obtained from the respective NOx sensor elements. An endurance test was performed on the obtained sample sensor. The NOx sensor element (1) corresponds to the test piece (1), and the NOx sensor elements (2), (3) and (4) are the test pieces (2) and (3), respectively. And it was manufactured corresponding to (4).

  And the result of the endurance test about each obtained sample sensor was shown in FIG. As is apparent from the results of FIG. 15, in the case of using the NOx sensor element (4) according to the comparative example, the sensitivity gradually increases from the start of durability to 700 hours, but after 700 hours, the sensitivity is It turns out that the sensitivity decreased to -25% after 1200 hours of durability. On the other hand, in the case of using the NOx sensor elements (1) to (3) according to the present invention, the sensitivity change rate is maintained within ± 3% up to durability of 2100 hours at any level. It showed a clear advantage over the NOx sensor element (4) of the comparative example.

It is longitudinal cross-sectional explanatory drawing which shows an example of the NOx sensor element according to this invention. FIG. 2 is a partial explanatory view showing a reduced form of a section taken along the line II-II in FIG. FIG. 3 is an explanatory view taken along the line III-III in FIG. 2, schematically showing an enlarged view of a measurement electrode and an electrode protective layer provided in a second space of the NOx sensor element shown in FIG. 1. It is explanatory drawing which shows the planar form of the heater element used in the NOx sensor element shown by FIG. It is sectional explanatory drawing which shows an example of the process of forming an electrode protective layer with the screen printing technique using a metal mask according to this invention. It is process explanatory drawing which shows the manufacturing process of the test piece produced in Example 1 in a plane form. FIG. 7 is an enlarged partial explanatory view showing a longitudinal section (corresponding to a VII-VII section in FIG. 6) of the laminated printed matter obtained in Example 1. It is explanatory drawing which shows the test piece (before baking) obtained in Example 1, Comprising: (a) is the longitudinal cross-sectional expanded partial explanatory drawing, and is equivalent to the schematic which shows the VIII-VIII cross section in (b). (B) is a plane explanatory view of such a test piece. 2 is a SEM cross-sectional photograph showing a vertical cross section of a test piece A obtained in Example 1. FIG. 2 is a SEM cross-sectional photograph showing a vertical cross section of a test piece B obtained in Example 1. FIG. 4 is a graph showing the durability test results obtained in Example 1. 4 is a graph showing the durability test results obtained in Example 2. 10 is a graph showing the durability test results obtained in Example 3. It is a graph which shows the endurance test result obtained in Example 4. 10 is a graph showing the durability test results obtained in Example 5.

Explanation of symbols

2 Sensor elements 4a to 4f Solid electrolyte layer 6 Reference air introduction passage 8 Buffer space 10 First space 12 Second space 14 Clogging prevention space 16 Gas inlet 18 First partition 20 First diffusion Rate limiting passage 22 Second partition wall 24 Second diffusion rate limiting passage 26 Inner pump electrode 26a Ceiling electrode portion 26b Lower electrode portion 26c Side electrode portion 28 Outer pump electrode 29 Porous protective layer 30 Third partition wall 32 Third Diffusion-controlled passage 34 Auxiliary pump electrode 36 Measurement (use) electrode 38 Electrode protective layer 38a Upper bottom portion 38b Thickness changing portion 39 Reference electrode 40 Porous alumina layer 42 Heater layer 44 Heater element 44a Heat generating portion 44b Current supply lead portion 44c Resistance detection Lead portion 45 Pressure dissipation hole 46 Through hole 47 Connector pad 50 Main pump cell 52 Oxygen partial pressure detection sensor for main pump cell control 54, 60, 66 Variable power supply 56 Auxiliary pump cell 58 Auxiliary pump cell control oxygen partial pressure detection cell 62 Measurement pump cell 64 Measurement pump cell control oxygen partial pressure detection cell 68 Sensor cells 70, 80, 92 Green sheets 72, 82 Measurement electrode formation Layer 74 Metal mask 76 Plastic squeegee 78 Electrode protective layer forming layer 84 Insulating layer 86 Electrode lead 88 Electrode lead upper insulating layer 90 Electrode protective layer forming part

Claims (24)

A measurement electrode composed of a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing NOx components in a gas to be measured, and a porous ceramic layer provided on the measurement electrode so as to cover it In the NOx measurement electrode part structure in which the electrode protective layer is formed by screen printing,
Such an electrode protective layer has at least a flat upper bottom portion extending horizontally in the horizontal direction on the measurement electrode, and a height change portion in which the height gradually decreases from both ends of the upper bottom portion to the sides of the measurement electrode. A NOx measuring electrode part structure formed in a trapezoidal cross-sectional shape.   2. The NOx measurement according to claim 1, wherein the upper bottom portion in the trapezoidal cross-sectional shape of the electrode protective layer is protruded laterally from both end portions of the measurement electrode so as to be larger than the width of the measurement electrode. Electrode structure.   The NOx measurement electrode part according to claim 1 or 2, wherein the electrode protective layer is formed such that the width of the upper base of the trapezoidal cross-sectional shape is 1.1 times or more the width of the measurement electrode. Construction.   The electrode protective layer is 0.9 to 1.0 in the sum of the thickness of the measurement electrode and the thickness of the electrode protective layer thereon at the side positions 80 μm apart from both ends of the measurement electrode. The NOx measurement electrode part structure according to any one of claims 1 to 3, having a thickness that is a multiplied value.   The thickness of the measurement electrode is 15 to 35 μm, the thickness of the electrode protective layer formed on the measurement electrode is 20 to 45 μm, and the sum of the thickness of the measurement electrode and the thickness of the electrode protective layer is 5. The NOx measurement electrode part structure according to claim 1, wherein the NOx measurement electrode part structure is configured to be 70 μm or less.   6. The NOx measurement electrode part structure according to claim 1, wherein the electrode protective layer is formed in a size obtained by adding 200 to 600 μm to the width of the measurement electrode. The electrode metal material that provides the cermet constituting the measurement electrode is a noble metal, while the ceramic material that provides the cermet contains 5.5 to 8.5 mol% of Y ₂ O ₃ in Y ₂ O ₃ stabilized ZrO _2. The NOx measurement electrode part structure according to any one of claims 1 to 6, which is a material.   The electrode metal material that provides the cermet constituting the measurement electrode is a noble metal, and the noble metal is Pt or an alloy of Pt and Rh, and Pt: Rh = 100 to 40 wt%: 0 to 60 wt% The NOx measurement electrode part structure according to any one of claims 1 to 7, wherein Pt and Rh are used so as to satisfy the ratio.   The electrode metal material that provides the cermet constituting the measurement electrode is a noble metal, and the ratio (vol%) between the noble metal and the ceramic material is in a range of 65/35 to 40/60. The NOx measurement electrode part structure according to any one of claims 1 to 8.   The measurement electrode and the electrode protective layer are integrally formed on a bottom surface in a void formed by being surrounded by a solid electrolyte, and a side wall made of a solid electrolyte that partitions and forms the void. The NOx measurement electrode part structure according to any one of claims 1 to 9, wherein the NOx measurement electrode part structure is disposed via a space between the side walls without contact. A measurement electrode composed of a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing NOx components in a gas to be measured, and a porous ceramic layer provided on the measurement electrode so as to cover it In the NOx measurement electrode part structure in which the electrode protective layer is formed by screen printing,
The noble metal is used as the electrode metal material for providing the cermet constituting the measurement electrode, while Y ₂ O ₃ stabilization containing 5.5 to 8.5 mol% of Y ₂ O ₃ is used as the ceramic material for providing the cermet. A NOx measuring electrode part structure comprising a ZrO ₂ material.   The method for forming the NOx measurement electrode part structure according to any one of claims 1 to 10, wherein the electrode protective layer is formed on the measurement electrode by a screen printing technique using a metal mask. A method for forming a NOx measurement electrode part structure comprising a step.   An electrochemical cell comprising the NOx measurement electrode part structure according to any one of claims 1 to 11 provided on a predetermined solid electrolyte body. A measurement electrode composed of a cermet of an electrode metal material and a ceramic material capable of reducing or decomposing NOx components in a gas to be measured, and a porous ceramic layer provided on the measurement electrode so as to cover it An NOx measurement electrode part structure composed of an electrode protective layer is included in an electrochemical cell formed on a predetermined solid electrolyte body, and the NOx component is reduced or decomposed at the measurement electrode. By measuring the amount of oxygen generated by the reduction or decomposition of the NOx component, a NOx sensor element for obtaining the concentration of the NOx component in the measured gas is obtained.
12. The NOx sensor element, wherein the NOx measurement electrode part structure is configured by the NOx measurement electrode part structure according to any one of claims 1 to 11.   It has a multilayered structure formed by laminating and integrating a plurality of solid electrolyte layers, and has a longitudinal element shape. Inside, from the element tip side to the element base side, a buffer space, a first space, The two vacant spaces are separately arranged, and an external gas to be measured is introduced into the buffer vacant space from the gas inlet formed on the tip side of the element through the first diffusion rate controlling means. And the atmosphere in the buffer space is introduced into the first space through the second diffusion rate-limiting means, and the atmosphere in the first space is further introduced into the second space through the third diffusion rate-limiting means. In the NOx sensor element configured to be guided into the site, the NOx measurement electrode unit structure according to any one of claims 1 to 11 is integrally formed on the wall surface of the second space. NOx sensor element characterized by being formed   The first inner pump electrode and the first outer pump electrode formed inside and outside the first space, and oxygen contained in the atmosphere introduced from the buffer space, the oxygen between the pump electrodes The NOx sensor element according to claim 15, wherein main pump means for performing a pumping process based on a control voltage applied to the NOx sensor element is provided.   The first inner pump electrode in the main pump means includes a ceiling electrode portion formed on the ceiling surface of the first cavity, a bottom electrode portion formed on the bottom surface, and those formed on the side wall surface. The NOx sensor element according to claim 16, which has a tunnel-type electrode structure including a side electrode portion connecting the ceiling electrode portion and the bottom electrode portion.   It has a pair of auxiliary pump electrodes formed inside and outside the second cavity, and oxygen contained in the atmosphere introduced from the first cavity is applied between the pair of auxiliary pump electrodes. The NOx sensor element according to any one of claims 15 to 17, wherein auxiliary pump means for performing pumping processing based on the auxiliary pump voltage is provided.   The auxiliary pump electrode formed and arranged in the second space is formed on the ceiling electrode portion formed on the ceiling surface of the second space, the bottom electrode portion formed on the bottom surface, and the side wall surface. 19. The NOx sensor element according to claim 18, wherein the NOx sensor element has a tunnel-type electrode structure including a side electrode portion connecting the ceiling electrode portion and the bottom electrode portion.   A clogging prevention space that opens outward is formed at the tip of the element, and the opening of the clogging prevention space constitutes the gas introduction port, while the clogging prevention space and the buffering space are formed. 20. The NOx sensor element according to any one of claims 15 to 19, wherein the first diffusion rate-limiting means is provided between the first and second locations.   21. The NOx sensor element according to any one of claims 15 to 20, wherein the first and second diffusion rate-limiting means are each formed in a slit shape having a gap of 10 μm or less.   A porous ceramic layer is integrally formed on the first outer pump electrode formed outside the first cavity, and the porous ceramic layer is formed of the first outer pump electrode. The NOx sensor element according to any one of claims 15 to 21, wherein the NOx sensor element is entirely covered.   In the laminated structure of the plurality of solid electrolyte layers, a heater layer for electrically heating the element is laminated and integrated. At least the first space and the second space are heated by the heater layer. The NOx sensor element according to any one of claims 15 to 22, wherein the solid electrolyte layer portion defining the void can be heated to a predetermined temperature. The heater layer includes a heater element having a heat generating portion, a current supply lead connected to the heat generating portion, and a resistance detection lead connected to a connection portion between the heat generating portion and the current supply lead. From the lead resistance detected using the resistance detection lead: R _L and the overall heater resistance detected using the current supply lead: Ra, the following formula:
R _H = Ra-2 × R _L
24. The NOx sensor element according to claim 23, wherein the voltage applied to the heater element is controlled so that the resistance: R _H of the heat generating portion of the heater element is constant according to.

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