JP2022113128A - Sensor element of NOx sensor - Google Patents

Abstract

To suppress a decline in sensitivity of an NOx sensor attributable to the oxidation of a measurement electrode.SOLUTION: A sensor element comprises: a base unit composed of a solid electrolyte of oxygen ion conduction type; at least one internal void into which gas to be measured is introduced; an internal electrode arranged facing the internal void; and at least one pump cell composed of an out-of-void pump electrode arranged at a point other than the internal void and a portion of the base unit existing between two electrodes. The internal electrode is composed of noble metal, a solid electrolyte, and a pore. A degree of area boundary variation which is a degree of variation in a thickness direction of the element at a boundary of the internal electrode between a first area composed of the base unit or the solid electrolyte continuous from the base unit and a second area occupied by the noble metal and pore is 0.5 μm or more and 6.5 μm or less.SELECTED DRAWING: Figure 2

Description

TECHNICAL FIELD The present invention relates to a sensor element of a limiting current type NOx sensor, and more particularly to a configuration of measurement electrodes of the sensor element.

Oxygen-ion conductive solid electrolyte ceramics such as zirconia (ZrO ₂ ) have been used as substrates for measuring NOx concentrations in gases to be measured such as combustion gases and exhaust gases in internal combustion engines such as automobile engines. NOx sensors forming a sensor element are known (see, for example, US Pat.

The sensor element (NOx sensor element) of such a NOx sensor comprises various electrodes (pump electrode, measurement electrode, reference electrode, etc.). These electrodes are porous cermet electrodes made of a composite material of a noble metal as a catalyst and zirconia as an electrolyte and having a porous structure containing a large number of pores. As the catalyst noble metal, Pt and Pt to which a small amount of other substance (for example, Rh or other noble metal) is added are used. The NOx sensor element is heated to a relatively high temperature (600° C. to 900° C.) to drive the sensor element, in order to utilize the catalytic reaction of the catalyst precious metal used for the electrode and the oxygen ion conductivity of the zirconia used for the substrate during its operation. used in

Gases to be measured, such as engine exhaust gases, usually contain _O2 . Therefore, in the NOx sensors disclosed in Patent Documents 1 and 2, O ₂ in the gas to be measured introduced into the NOx sensor element is removed by an electrochemical pump cell provided in the NOx sensor element. (separated from NOx), NOx is decomposed into _O2 and _N2 using the catalytic reaction of the measuring electrode, and based on the magnitude of the oxygen ion current flowing through the measuring pump cell, NOx concentration is measured.

Japanese Patent No. 2885336 WO2019/188613

When the measurement electrode is exposed to _O2 , Pt or Pt/Rh on the electrode reacts with _O2 to produce oxides such as PtO, _PtO2 and _RhO2 . When the noble metal of the electrode is reduced by the formation of such oxides, the catalytic reactivity of the measurement electrode is lowered. In addition, since these oxides have a lower vapor pressure than Pt, they are easier to evaporate at a lower temperature than Pt. Such evaporation reduces the three-phase interface of catalyst/electrolyte/pores and the two-phase interface of catalyst/electrolyte in the measurement electrode, which is also a factor in reducing the catalytic reactivity of the measurement electrode.

As described above, the gas to be measured that reaches the measuring electrode has oxygen removed in advance, but even so, a small amount of oxygen remains in the gas to be measured. In addition, O ₂ generated by decomposition of NOx may not be ionized quickly and may remain in the vicinity of the measurement electrode.

Therefore, if the NOx sensor continues to be used continuously, the NOx sensitivity will gradually decrease due to the decrease in the catalytic reactivity of the measuring electrode.

More specifically, the mass transfer in the vicinity of the measuring electrode during measurement will be described in more detail. First, O ₂ and NOx in the gas to be measured that have reached the measuring electrode move through the pores of the measuring electrode (this is referred to as the mass transfer process). (referred to as When these O ₂ and NOx come into contact with the catalyst metal in the pores, Pt, which is the main component of the catalyst metal, decomposes NO into N ₂ and O ₂ and further takes electrons from O ₂ to generate oxygen ions. The generated oxygen ions are taken from Pt into the solid electrolyte zirconia at the catalyst/electrolyte two-phase interface, and move in the zirconia due to the potential difference generated between the measurement electrode and the external electrode when voltage is applied by an external power supply. (This is called a charge transfer process).

However, in this series of processes, if there are oxygen ions that cannot be taken into zirconia from the catalyst metal and oxygen before ionization (these are collectively referred to as surplus oxygen), the catalyst metal, mainly Pt, will be removed from the inside and surface of the electrode. , PtO, PtO ₂ and the like are produced by the excess oxygen. These oxides then gradually evaporate.

The inventor of the present invention believes that by reducing the excess oxygen that cannot be taken into zirconia, the evaporation of oxides from the measurement electrode can be suppressed, and the decrease in sensitivity of the NOx sensor can be suppressed. have arrived at the present invention.

Note that Patent Document 2 focuses on the fact that impurities present in the electrode when oxidizing Pt in the measurement electrode further lower the evaporation temperature of the oxide, and provides a gettering layer for gettering such impurities to oxidize Pt. A mode for suppressing the evaporation of substances is disclosed. However, Patent Literature 2 neither discloses nor suggests suppression of oxide evaporation by reducing surplus oxygen.

The present invention has been made in view of the above problems, and an object thereof is to provide a NOx sensor in which a decrease in sensitivity due to oxidation of the measurement electrode is suppressed even when used continuously. do.

In order to solve the above-mentioned problems, a first aspect of the present invention is a sensor element of a limiting current type NOx sensor, comprising: a base portion made of an oxygen ion conductive solid electrolyte; an internal electrode facing the at least one internal cavity; and an external pump electrode positioned outside the at least one internal cavity; at least one pump cell composed of a portion of the base portion existing between the internal electrode and the external pump electrode, wherein the internal electrode is composed of a noble metal, the solid electrolyte, and pores. and a region boundary, which is the degree of variation in the element thickness direction of the boundary between the base portion or the solid electrolyte continuous from the base portion and the second region occupied by the noble metal and pores in the internal electrode. It is characterized in that the degree of variation is 0.5 μm or more and 6.5 μm or less.

A second aspect of the present invention is the sensor element of the NOx sensor according to the first aspect, wherein the at least one internal cavity is introduced with the gas to be measured whose oxygen concentration has been adjusted in advance. an inner measuring cavity, said inner electrode being a measuring electrode arranged in said inner measuring cavity, said at least one pump cell comprising said measuring electrode, said outer pump electrode and said base portion; and a portion existing between the measurement electrode and the pump electrode outside the cavity.

A third aspect of the present invention is the sensor element of the NOx sensor according to the second aspect, wherein the measurement electrode comprises an upper layer comprising the noble metal, the solid electrolyte, and pores; a lower layer made of a solid electrolyte, wherein the volume ratio of the solid electrolyte in the upper layer is 20% to 40%, and the volume ratio of the solid electrolyte in the lower layer is 50% to 60%. It is characterized by

A fourth aspect of the present invention is the sensor element of the NOx sensor according to the third aspect, wherein the thickness ratio between the upper layer and the lower layer is in the range of 95:5 to 10:90. It is characterized by

A fifth aspect of the present invention is the sensor element of the NOx sensor according to the fourth aspect, wherein the thickness ratio between the upper layer and the lower layer is in the range of 90:10 to 30:70. It is characterized by

A sixth aspect of the present invention is the sensor element of the NOx sensor according to any one of the first to fifth aspects, characterized in that the area boundary variation degree is 1 μm or more and 5 μm or less. do.

According to the first to sixth aspects of the present invention, deterioration of the measuring electrode due to the presence of surplus oxygen that cannot be taken into the measuring electrode is preferably suppressed. As a result, a NOx sensor is realized in which the deterioration of the measurement sensitivity is preferably suppressed even if the sensor is used continuously.

1 is a diagram schematically showing an example of the configuration of a gas sensor 100; FIG. 4 is a model diagram showing a partial cross section of the measuring electrode 44 along the element thickness direction. FIG. FIG. 10 is a diagram for explaining how to evaluate the degree of area boundary variation of the boundary between the first area and the second area in the element thickness direction; FIG. 10 is a model diagram of a partial cross section of a measurement electrode 44Z having a configuration in which three phases are mixed in the entire electrode. FIG. 4 is a diagram showing the flow of processing when manufacturing the sensor element 101. FIG. FIG. 7 is a diagram showing a procedure for forming a pattern that will eventually become a measurement electrode 44. FIG. It is a graph which shows the result of a continuous drive test.

<Schematic configuration of gas sensor>
FIG. 1 is a diagram schematically showing an example of the configuration of a gas sensor 100 according to this embodiment. The gas sensor 100 is a limiting current type NOx sensor that detects NOx with a sensor element 101 and measures its concentration. Moreover, the gas sensor 100 further includes a controller 110 that controls the operation of each part and specifies the NOx concentration based on the NOx current flowing through the sensor element 101 . FIG. 1 includes a vertical cross-section along the length of sensor element 101 .

The sensor element 101 comprises a first substrate layer 1, a second substrate layer 2, each made of zirconia (ZrO ₂ ), which is an oxygen ion conductive solid electrolyte (for example, made of yttria-stabilized zirconia (YSZ), etc.); Six solid electrolyte layers, namely, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, are stacked in this order from the bottom when viewed from the drawing. It is a flat plate-like (long plate-like) element. Also, the solid electrolyte forming these six layers is dense and airtight. In addition, hereinafter, the upper surface of each of these six layers in FIG. 1 may be simply referred to as the upper surface, and the lower surface thereof may simply be referred to as the lower surface. In addition, the entire portion of the sensor element 101 made of solid electrolyte is collectively referred to as a base portion.

The sensor element 101 is manufactured by, for example, performing predetermined processing and circuit pattern printing on ceramic green sheets corresponding to each layer, laminating them, and firing them to integrate them.

Between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 at one tip of the sensor element 101, there are provided a first diffusion control portion 11 that also serves as a gas inlet 10, and a buffer space. 12, a second diffusion rate-limiting portion 13, a first internal space 20, a third diffusion rate-limiting portion 30, a second internal space 40, a fourth diffusion rate-limiting portion 60, and a third internal space 61. are formed adjacent to each other in a manner communicating in this order.

The buffer space 12 , the first internal space 20 , the second internal space 40 , and the third internal space 61 are formed by hollowing out the spacer layer 5 so that the upper part of the second solid electrolyte layer 6 is formed. It is a space (region) inside the sensor element 101 defined by the bottom surface, the bottom surface of the first solid electrolyte layer 4 , and the side surface of the spacer layer 5 . Similarly, the gas introduction port 10 is also provided separately from the first diffusion control portion 11 by hollowing out the spacer layer 5 on the front end surface (left end in the drawing) of the sensor element 101. good too. In such a case, the first diffusion control portion 11 is formed inside and adjacent to the gas introduction port 10 .

Each of the first diffusion rate-controlling part 11, the second diffusion rate-controlling part 13, the third diffusion rate-controlling part 30, and the fourth diffusion rate-controlling part 60 has two horizontally long (openings in the direction perpendicular to the drawing). direction). A portion from the gas introduction port 10 to the third internal space 61 is also called a gas circulation portion.

Further, at a position farther from the tip side than the gas flow part, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, the side portion is partitioned by the side surface of the first solid electrolyte layer 4. A reference gas introduction space 43 is provided at a position where the reference gas is introduced. For example, atmospheric air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43 . Also, the atmosphere introduction layer 48 is formed so as to cover the reference electrode 42 .

The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, is connected to the reference gas introduction space 43 around it. An atmosphere introduction layer 48 is provided. Further, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 using the reference electrode 42 .

In the gas flow section, the gas introduction port 10 (first diffusion control section 11) is a portion that is open to the external space, and the gas to be measured enters the sensor element 101 from the external space through the gas introduction port 10. It is designed to be taken in.

The first diffusion control portion 11 is a portion that imparts a predetermined diffusion resistance to the taken gas under measurement.

The buffer space 12 is a space provided for guiding the gas to be measured introduced from the first diffusion rate controlling section 11 to the second diffusion rate controlling section 13 .

The second diffusion control portion 13 is a portion that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20 .

When the gas to be measured is introduced from the outside of the sensor element 101 into the first internal cavity 20, the pressure fluctuation of the gas to be measured in the external space (the pulsation of the exhaust pressure if the gas to be measured is the exhaust gas of an automobile) ) is not introduced directly into the first internal space 20, but rather into the first diffusion rate-determining portion 11, the buffer space 12, the second After concentration fluctuations of the gas to be measured are canceled out through the diffusion control section 13 , the gas is introduced into the first internal cavity 20 . As a result, fluctuations in the concentration of the gas to be measured introduced into the first internal cavity 20 are almost negligible.

The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion control section 13 . The oxygen partial pressure is adjusted by operating the main pump cell 21 .

The main pump cell 21 includes an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, and an upper surface of the second solid electrolyte layer 6 ( An outer (outside the cavity) pump electrode 23 provided in a manner exposed to the external space in a region corresponding to the ceiling electrode portion 22a on one main surface of the sensor element 101, and a second solid body sandwiched between these electrodes. It is an electrochemical pump cell constituted by an electrolyte layer 6 .

The inner pump electrode 22 is formed on the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that partition the first internal cavity 20. As shown in FIG. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20, and a bottom electrode portion 22a is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. An electrode portion 22b is formed. The ceiling electrode portion 22a and the bottom electrode portion 22b are connected by conductive portions (not shown) provided on the side wall surfaces (inner surfaces) of the spacer layer 5 constituting both side wall portions of the first internal space 20. ).

The ceiling electrode portion 22a and the bottom electrode portion 22b are provided in a rectangular shape in plan view. However, a mode in which only the ceiling electrode portion 22a or only the bottom electrode portion 22b is provided may be employed.

The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes. In particular, the inner pump electrode 22 that comes into contact with the gas to be measured is formed using a material that weakens the ability to reduce NOx components in the gas to be measured. For example, a cermet electrode of ZrO ₂ and an Au—Pt alloy having a porosity of 5% to 40% and containing 0.6 wt % to 1.4 wt % of Au is formed to a thickness of 5 μm to 20 μm. The weight ratio of the Au—Pt alloy and ZrO ₂ may be about Pt:ZrO ₂ =7.0:3.0 to 5.0:5.0.

On the other hand, the outer pump electrode 23 is formed as a cermet electrode of Pt or its alloy and ZrO ₂ and has a rectangular shape in plan view.

In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by the variable power supply 24, and a positive or negative voltage is applied between the inner pump electrode 22 and the outer pump electrode 23. By flowing the main pump current Ip0 in the direction, it is possible to pump the oxygen in the first internal space 20 to the external space or pump the oxygen in the external space into the first internal space 20 . The pump voltage Vp0 applied between the inner pump electrode 22 and the outer pump electrode 23 in the main pump cell 21 is also called the main pump voltage Vp0.

In order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 , the third substrate layer 3 and the reference electrode 42 constitute a main sensor cell 80 which is an electrochemical sensor cell.

By measuring the electromotive force V0 in the main sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known.

Further, the main pump current Ip0 is controlled by the controller 110 feedback-controlling the main pump voltage Vp0 so that the electromotive force V0 is constant. As a result, the oxygen concentration in the first internal space 20 is maintained at a predetermined constant value.

The third diffusion rate controlling section 30 applies a predetermined diffusion resistance to the gas under measurement whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20, thereby reducing the gas under measurement. It is a portion that leads to the second internal space 40 .

The second internal space 40 is provided as a space for further adjusting the oxygen partial pressure in the gas to be measured introduced through the third diffusion control section 30 . Such oxygen partial pressure is adjusted by operating the auxiliary pump cell 50 . In the second internal space 40, the oxygen concentration of the gas to be measured is adjusted with even higher precision.

In the second internal space 40, after the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, the gas to be measured introduced through the third diffusion control section 30 is further subjected to the auxiliary pump cell 50. The oxygen partial pressure is adjusted by

The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40, and an outer pump electrode 23 (outer pump electrode 23 , the sensor element 101 and a suitable external electrode), and the second solid electrolyte layer 6, an auxiliary electrochemical pump cell.

The auxiliary pump electrode 51 is arranged in the second internal space 40 in the same manner as the inner pump electrode 22 provided in the first internal space 20 described above. That is, the ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal cavity 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal cavity 40. , a bottom electrode portion 51b is formed. The ceiling electrode portion 51a and the bottom electrode portion 51b are rectangular in plan view, and are conductive portions provided on the side wall surfaces (inner surfaces) of the spacer layer 5 constituting both side wall portions of the second internal cavity 40. (illustration omitted).

As with the inner pump electrode 22, the auxiliary pump electrode 51 is also made of a material having a weakened ability to reduce NOx components in the gas to be measured.

In the auxiliary pump cell 50, under the control of the controller 110, by applying a desired voltage (auxiliary pump voltage) Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, Oxygen in the atmosphere can be pumped into the external space or pumped into the second internal cavity 40 from the external space.

In order to control the oxygen partial pressure in the atmosphere inside the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte The layer 4 and the third substrate layer 3 constitute an auxiliary sensor cell 81 which is an electrochemical sensor cell.

The auxiliary pump cell 50 performs pumping with the variable power source 52 whose voltage is controlled based on the electromotive force V1 corresponding to the oxygen partial pressure in the second internal space 40 detected by the auxiliary sensor cell 81 . Thereby, the oxygen partial pressure in the atmosphere inside the second internal cavity 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

Along with this, the auxiliary pump current Ip1 is used for controlling the electromotive force of the main sensor cell 80. FIG. Specifically, the auxiliary pump current Ip1 is input to the main sensor cell 80 as a control signal, and is introduced into the second internal space 40 from the third diffusion rate-determining section 30 by controlling the electromotive force V0 thereof. It is controlled so that the gradient of the oxygen partial pressure in the gas to be measured is always constant. When used as a NOx sensor, the main pump cell 21 and the auxiliary pump cell 50 work to keep the oxygen concentration in the second internal cavity 40 at a constant value of approximately 0.001 ppm.

The fourth diffusion rate control section 60 applies a predetermined diffusion resistance to the gas under measurement whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal space 40, thereby reducing the gas under measurement. It is a portion that leads to the third internal space 61 .

The third internal space 61 is provided as a space for performing processing related to measurement of nitrogen oxide (NOx) concentration in the gas to be measured introduced through the fourth diffusion control section 60 . The NOx concentration is measured by operating the measuring pump cell 41 in the third internal cavity 61 . Since the gas to be measured whose oxygen concentration has been adjusted with high precision in the second internal space 40 is introduced into the third internal space 61, the gas sensor 100 can measure the NOx concentration with high precision.

The measurement pump cell 41 measures the NOx concentration in the gas to be measured within the third internal cavity 61 . The measurement pump cell 41 includes a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61 and at a position spaced apart from the third diffusion control section 30, an outer pump electrode 23, It is an electrochemical pump cell composed of a second solid electrolyte layer 6 , a spacer layer 5 and a first solid electrolyte layer 4 .

The measurement electrode 44 is a porous cermet electrode of noble metal and solid electrolyte. For example, it is formed as a cermet electrode of Pt or an alloy of Pt and other noble metal such as Rh and ZrO ₂ which is a constituent material of the sensor element 101 . The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere inside the third internal cavity 61 .

In the measurement pump cell 41, under the control of the controller 110, oxygen generated by decomposition of NOx in the atmosphere around the measurement electrode 44 can be pumped out, and the amount of oxygen generated can be detected as the pump current Ip2.

However, in the present embodiment, the measuring electrode 44 has a two-layer structure of a region without pores and a region with pores. The details will be described later.

In order to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, Together with the reference electrode 42, a measurement sensor cell 82, which is an electrochemical sensor cell, is formed. The variable power supply 46 is controlled based on the electromotive force V2 corresponding to the oxygen partial pressure around the measuring electrode 44 detected by the measuring sensor cell 82 .

NOx in the measured gas introduced into the third internal space 61 is reduced by the measuring electrode 44 (2NO→N ₂ +O ₂ ) to generate oxygen. The generated oxygen is pumped by the measurement pump cell 41. At this time, the voltage (measurement pump voltage) Vp2 of the variable power supply 46 is maintained so that the electromotive force V2 detected by the measurement sensor cell 82 is constant. is controlled. Since the amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of NOx in the gas to be measured, the NOx concentration in the gas to be measured is calculated using the pump current Ip2 in the measuring pump cell 41. It will happen. Hereinafter, such pump current Ip2 is also referred to as NOx current Ip2.

Further, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 are combined to constitute an oxygen partial pressure detecting means as an electrochemical sensor cell, the measurement electrode 44 can be It is possible to detect the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx components in the surrounding atmosphere and the amount of oxygen contained in the reference atmosphere, thereby determining the concentration of the NOx components in the gas to be measured. is also possible.

An electrochemical sensor cell 83 is composed of the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42. The oxygen partial pressure in the gas to be measured outside the sensor can be detected from the electromotive force Vref obtained by the sensor cell 83 .

The sensor element 101 further includes a heater section 70 that plays a role of temperature adjustment for heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte forming the base section.

The heater section 70 mainly includes a heater electrode 71, a heater element 72, a heater lead 72a, a through hole 73, a heater insulating layer 74, and a heater resistance detection lead (not shown in FIG. 1). . Also, the heater portion 70 is embedded in the base portion of the sensor element 101 except for the heater electrode 71 .

The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

The heater element 72 is a resistance heating element provided between the second substrate layer 2 and the third substrate layer 3 . The heater element 72 is supplied with power from a heater power supply (not shown) provided outside the sensor element 101, not shown in FIG. Fever. The heater element 72 is made of Pt or made mainly of Pt. The heater element 72 is embedded in a predetermined range on the side of the sensor element 101 provided with the gas circulation part so as to face the gas circulation part in the element thickness direction. The heater element 72 is provided so as to have a thickness of approximately 10 μm to 20 μm.

In the sensor element 101, by causing the heater element 72 to generate heat by passing a current through the heater element 72 through the heater electrode 71, each part of the sensor element 101 can be heated to a predetermined temperature and kept warm. there is Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and electrodes in the vicinity of the gas flow portion is about 700.degree. C. to 900.degree. Such heating increases the oxygen ion conductivity of the solid electrolyte forming the base portion of the sensor element 101 . The heating temperature by the heater element 72 when the gas sensor 100 is used (when the sensor element 101 is driven) is called the sensor element driving temperature.

The degree of heat generation (heater temperature) by the heater element 72 is grasped by the magnitude of the resistance value (heater resistance) of the heater element 72 .

Although not shown in FIG. 1, a single-layer or multi-layer porous layer covering the sensor element 101 is provided on the outer periphery of a predetermined range on the side of one tip portion of the sensor element 101 (on the left end side in the drawing). A mode in which a thermal shock resistant protective layer is further provided may be used. The purpose of the thermal shock protection layer is to prevent cracks from occurring in the sensor element 101 due to thermal shock caused by moisture contained in the gas to be measured adhering to and condensing on the sensor element 101 when the gas sensor 100 is used. , are provided for the purpose of preventing poisoning substances mixed in the gas to be measured from entering the inside of the sensor element 101 . Note that a layered void (void layer) may be formed between the sensor element 101 and the thermal shock protection layer.

<Detailed configuration of measurement electrodes>
Next, the configuration of the measurement electrodes 44 provided in the sensor element 101 having the configuration described above will be described in more detail. FIG. 2 is a model diagram showing a partial cross section of the measuring electrode 44 along the element thickness direction. Broken lines La and Lb shown in FIG. 2 are attached for easy understanding.

In FIG. 2, the area below the dashed line La is the first solid electrolyte layer 4 made of a solid electrolyte (zirconia), and the measurement electrode 44 is provided on the first solid electrolyte layer 4 . Above the measuring electrode 44 is a third internal space 61 (more specifically, a portion of the third internal space 61 not occupied by the measuring electrode 44; 61).

As described above, the measurement electrode 44 is a porous cermet electrode made of a noble metal such as Pt or its alloy with Rh and a solid electrolyte (zirconia). Therefore, as shown in FIG. 2, the measurement electrode 44 has a portion made of noble metal NM that is white when viewed from the drawing, a portion made of the solid electrolyte SE that is gray (or light gray) when viewed from the drawing, and a black portion (or dark gray) when viewed from the drawing. ) and the portion composed of the pores CV have a mixed configuration. In addition, in FIG. 2, the first solid electrolyte layer 4 is also gray when viewed from the drawing, like the solid electrolyte SE. This corresponds to both being made of zirconia. In addition, the third inner space 61 is also black in the drawing, like the pores CV, which means that both of them are spaces or voids.

However, the measurement electrode 44 according to the present embodiment does not have a configuration in which the three portions (phases) of the noble metal NM, the solid electrolyte SE, and the pores CV are randomly mixed throughout, but the noble metal NM and the solid electrolyte A lower layer (also referred to as a two-phase region) 44L in which only SE is randomly mixed and no pores CV exist, and an upper layer (also referred to as a three-phase region) 44U in which noble metal NM, solid electrolyte SE, and pores CV are randomly mixed. It has a two-layer structure. In FIG. 2, the boundary between the lower layer 44L and the upper layer 44U is indicated by a dashed line Lb.

From another point of view, the broken line La indicates the lowest end of the existence range of the noble metal NM in the thickness direction of the measurement electrode 44, and the broken line Lb indicates the lowest end of the existence range of the pores CV in the thickness direction of the measurement electrode 44. is shown. In other words, the dashed line La can be said to be the boundary between the region where the noble metal NM exists and the region where the noble metal NM does not exist in the thickness direction of the measurement electrode 44, and the dashed line Lb represents the thickness of the measurement electrode 44. It can also be said to be the boundary in the direction between the region where the pore CV exists and the region where the pore CV does not exist.

In the following description, of the solid electrolyte SE constituting the measurement electrode 44, the portion of the solid electrolyte SE of the sensor element 101 that is continuous with the base portion (the portion that is continuous with the base portion beyond the dashed line La) will be referred to as the continuous region RE. called. Further, a portion of the continuous region RE belonging to the lower layer 44L (a portion located between the dashed lines La and Lb) will be referred to as a lower continuous region REL, and a portion of the continuous region RE belonging to the upper layer 44U (a portion located between the dashed lines Lb and the third internal cavity 61) is called an upper continuous region REU.

As described above, the measurement electrode 44 included in the sensor element 101 according to the present embodiment is provided as a porous cermet electrode in which the noble metal NM, the solid electrolyte SE, and the pores CV are mixed as a whole. The three phases are mixed only in the upper layer 44U provided on the upper surface side of the electrode, and below it is the lower layer 44L having a two-phase structure in which no pores CV exist.

In other words, in the measurement electrode 44, the boundary (broken line La) between the base portion made of the solid electrolyte (more specifically, the first solid electrolyte layer 4) and the measurement electrode 44, the region where the pores CV exist and the existence The boundary (broken line Lb) with the area where the sintering is not performed is different.

<Example of specifying the boundary position>
Next, the boundary position between the region where the noble metal NM exists and the region where the noble metal NM does not exist, which is also the lowest end of the range where the noble metal NM exists in the thickness direction of the measurement electrode 44 having the configuration as described above, and the position of the pore CV. An example of how to specify each boundary position (broken lines La, Lb) when evaluating the boundary position between the region where the pore CV exists and the region where the pore CV does not exist, which is also the lowest end of the existence range to explain.

First, a cross-sectional image of the measurement electrode 44 along the thickness direction as shown in FIG. 2 is captured by, for example, a SEM (scanning electron microscope). At that time, the measuring electrode 44 is positioned as horizontally as possible in the captured image, and at least the entire range in the thickness direction of the lower layer 44L of the measuring electrode 44 and the area between the measuring electrode 44 and the first solid electrolyte layer 4 are arranged. The imaging range is set so as to include the vicinity of the boundary. From the viewpoint that it is necessary to clearly identify each interface of the three phases in the captured image obtained, the imaging magnification is preferably about 500 times to 1000 times. In this case, a cross-sectional image in the range of about 100 μm to 200 μm in the element longitudinal direction can be obtained.

Subsequently, the obtained captured image data is analyzed based on a known image processing technique, and the noble metal NM, the solid electrolyte SE (and the first solid electrolyte layer 4), the pores CV (and the third internal cavity 61) (all pixels corresponding to each) are specified. For example, when the captured image is an SEM image, it is possible to specify which phase each pixel of the captured image corresponds to from the difference in brightness.

Subsequently, a coordinate point (pixel position) that gives the lower end position of the noble metal NM in the captured image is specified based on the captured image data. In the case of FIG. 2, point A corresponds to this. Then, a straight line passing through the point A and parallel to the left-right direction of the captured image corresponds to the dashed line La indicating the lowest end of the existence range of the noble metal NM.

However, although the measurement electrodes 44 are set to be as horizontal as possible during imaging, the measurement electrodes 44 may be not a little inclined in the captured image obtained. Based on this point, a coordinate point at the lower end position (for example, point A in FIG. 2) and a coordinate point located next below (for example, point B in FIG. 2) are specified from the range of the noble metal NM in the captured image, The captured image is corrected so that the straight line passing through the points A and B is horizontal, and the straight line passing through the points A and B in the corrected captured image is the dashed line La may be

After the dashed line La is specified, subsequently, a coordinate point (pixel position) that gives the lower end position of the pore CV in the captured image (after correction) is specified based on the captured image data. In the case of FIG. 2, point C corresponds to this. Then, a straight line passing through the point C and parallel to the left-right direction of the captured image corresponds to the dashed line Lb indicating the lowest end of the existence range of the pore CV.

<Region Boundary Variation of Solid Electrolyte Region>
In the sensor element 101 according to the present embodiment, as described above, the continuous region RE, which is part of the solid electrolyte SE forming the measurement electrode 44, is continuous from the base portion made of the solid electrolyte. Most of them are lower continuous regions REL belonging to the lower layer 44L where no pores are present, but some are further continuous from the lower continuous regions REL and belong to the upper layer 44U as upper continuous regions REU. .

Since there are no pores CV in the lower layer 44L, the lower end portion of the lower layer 44L where the solid electrolyte SE exists all form a continuous region RE. As a result, in the sensor element 101 according to the present embodiment, the continuous region RE is significantly formed as compared with the conventional sensor element in which the pores CV exist up to the lower end portion of the lower layer 44L.

The fact that the continuous region RE is formed remarkably in the measurement electrode 44 in this way means that the solid electrolyte region (that is, the first 1 The boundary (hereinafter referred to as region Also referred to as a boundary) is not along the boundary (that is, broken line La) between the first solid electrolyte layer 4 and the measurement electrode 44, and varies significantly in the device thickness direction.

FIG. 3 shows an example of a model diagram of a partial cross section of the measurement electrode 44 shown in FIG. It is a figure for demonstrating a method.

As shown in FIG. 3, the x-axis is a straight line Lc that coincides with or is parallel to the dashed line La that indicates the lowest end of the existence range of the noble metal NM, and an arbitrary point x=x(i) on the x-axis is set as the starting point. , when a straight line is extended in the element thickness direction perpendicular to the dashed line La or Lc from the starting point, whichever of the lower end of the noble metal NM and the lower end of the pore CV reaches first, the area on the straight line Boundary point. When the region boundary varies significantly in the element thickness direction, the length d(i) of the line segment (shown with an arrow in FIG. 3) from the starting point to the region boundary varies depending on the position of the selected starting point. A different trend becomes pronounced. Since the pore CV exists only in the upper layer 44U, the location where the region boundary point is the intersection of the pore CV and the first region is a location where the first region particularly remarkably enters the second region. and can be grasped.

Based on the above, as shown in FIG. 3, when a large number of starting points x=x(1) to x(n) are taken at predetermined intervals along the straight line Lc and the corresponding area boundary points are specified for each, The standard deviation σ of the lengths d(1) to d(n) of line segments from the starting point to the region boundary point is an index of the degree of region boundary variation. That is, the larger the value of the standard deviation σ, the more the boundary between the first region and the second region varies significantly in the element thickness direction. is along the boundary between the first solid electrolyte layer 4 and the measurement electrode 44 (that is, the broken line La).

In specifying the distance d(i), the noble metal NM, which is entirely surrounded by the solid electrolyte SE and is not in contact with the pores CV, may be ignored. This is because the continuous region RE still exists above it in the element thickness direction.

Further, in FIG. 3, for convenience of explanation, the straight line Lc is different from the dashed line La, but they may coincide with each other. This is because the average value of the distance d(i) varies depending on the position of the straight line Lc, but the standard deviation σ does not depend on the position of the straight line Lc.

Furthermore, in FIG. 3, the intervals between the starting points are discrete for the convenience of illustration, but actually, it is preferable to set the starting points at finer intervals, for example, at intervals of 0.1 μm.

FIG. 4 is a diagram showing a state in which a model diagram of a partial cross section of a measurement electrode 44Z having a configuration in which three phases are randomly mixed in the entire electrode, shown for comparison, is used as an object for evaluation of the degree of area boundary variation. be. The arrangement position of the measurement electrode 44Z in the sensor element is the same as that of the measurement electrode 44. As shown in FIG. In the measurement electrode 44Z shown in FIG. 4 as well, the white portion when viewed from the drawing is the noble metal NM, the gray portion when viewed from the drawing is the solid electrolyte SE, and the black portion when viewed from the drawing is the pore CV.

In the measurement electrode 44Z, the boundary between the region where the pores CV exist and the region where the pores CV do not exist is substantially aligned with the boundary between the base portion made of the solid electrolyte (more specifically, the first solid electrolyte layer 4) and the measurement electrode 44. I am doing it. That is, a considerable portion of the base portion is in contact with the pores CV or noble metal NM. Further, although there is a continuous region RE where the solid electrolyte SE continues from the base portion, the degree of formation thereof is slight due to the presence of pores CV.

Therefore, in the case shown in FIG. 4, there is almost no difference between the lengths d(1) to d(n) of the line segments that can be obtained in the same manner as in the case of FIG. Therefore, the area boundary variation degree for the sensor element provided with the measurement electrode 44Z is smaller than the area boundary variation degree for the sensor element 101 provided with the measurement electrode 44 having the two-layer structure. .

This suggests that the area boundary variation degree takes a value that reflects the configuration of the measurement electrodes 44 .

<Relationship between configuration of measurement electrode and suppression of deterioration>
The measuring electrode 44 and the measuring electrode 44Z have in common the process of mass transfer within the electrodes. That is, NOx and a small amount of _O2 contained in the gas under measurement reaching the measuring electrode 44 or 44Z arranged in the third internal space 61 from the outside of the element move through the pores CV and reach the noble metal NM as a catalyst. Contact. Pt, which is the main component of the noble metal NM, decomposes NOx to generate _O2 , takes electrons from _O2 , and generates oxygen ions. Oxygen ions generated thereby are taken from Pt into the solid electrolyte SE at the two-phase interface between the noble metal NM and the solid electrolyte SE, and are generated between the measuring electrode 44 or 44Z and the outer pump electrode 23 in the measuring pump cell 41. It will move in the sensor element 101 due to the potential difference between them.

However, in the case of the measurement electrode 44Z which has a three-phase structure as a whole and therefore has a small degree of area boundary variation, the pores CV are present up to the vicinity of the first solid electrolyte layer 4 or adjacent to the first solid electrolyte layer 4. , and the existence ratio of the interface between the solid electrolyte SE and the pores CV is relatively high, while the existence ratio of the interface between the noble metal NM and the solid electrolyte SE is relatively low. For this reason, surplus oxygen that comes into contact with the noble metal but is not taken into the solid electrolyte is likely to be generated, and therefore, deterioration of the measurement electrode is likely to occur due to oxidation of the noble metal, which is mainly Pt, by surplus oxygen. It's becoming

On the other hand, in the case of the measurement electrode 44 which is composed of the lower layer 44L having a two-phase structure and the upper layer 44U having a three-phase structure and therefore has a large range boundary variation, the pores CV through which oxygen flows exist in the upper layer. 44U only, the solid electrolyte SE present in the lower layer 44L is almost the lower continuous region REL that is continuous from the base portion and is in contact only with the noble metal NM. Since the ratio of the two-phase interface between the noble metal NM and the solid electrolyte SE is high, and in particular, the interface existing in the lower layer 44L is only such a two-phase interface, the surplus oxygen contacts the noble metal NM and enters the pores CV. Retention and deterioration of the measurement electrode 44 are less likely to occur.

That is, in gas sensor 100 according to the present embodiment, deterioration of measurement electrode 44 due to the presence of excess oxygen is preferably suppressed. Therefore, even if the gas sensor 100 is used continuously, the deterioration of the measurement sensitivity is preferably suppressed.

It is preferable that the area boundary variation is set to 0.5 μm or more and 6.5 μm or less.

In this case, the rate of change of the NOx current Ip2 with respect to the initial value is suppressed to 6% or less when the gas sensor 100 is used continuously. That is, deterioration of the measurement sensitivity when the gas sensor 100 is continuously used is suppressed to 6% or less.

More preferably, the area boundary variation degree is 1 μm or more and 5 μm or less. In this case, the change rate of the NOx current Ip2 with respect to the initial value when the gas sensor 100 is continuously used, that is, the deterioration of the measurement sensitivity when the gas sensor 100 is continuously used is suppressed to about 3% or less. . In addition, the decrease in the pumping capacity of the measurement pump cell 41 due to a small number of sites for ionizing the gas to be measured, which can occur when the measurement electrode 44 has a large degree of area boundary variation and therefore a small number of pores, is also suppressed.

Also, preferably, the measuring electrode 44 has a thickness of 10 μm to 30 μm, and the thickness ratio t _U :t _L is in the range of 95:5 to 10:90. In other words, the distance tU from the top of the measurement electrode 44 to the lowest end (broken line Lb) of the existence range of the pores CV and the distance _tU from the bottom of the measurement electrode 44 to the lowest end of the existence range of the pores CV It also means that the ratio with the distance t _L is in the range of 95:5 to 10:90.

The volume ratio of the solid electrolyte SE in the upper layer 44U is set to 20% to 40%, and the volume ratio of the solid electrolyte SE in the lower layer 44L is set to 50% to 60%.

In addition, the configuration of the measurement electrode 44 with a large degree of area boundary variation, in which the solid electrolyte continuous from the first solid electrolyte layer 4 enters the noble metal portion, exhibits a so-called anchor effect. Therefore, the configuration of the measurement electrode 44 employed in the gas sensor 100 according to the present embodiment is also effective in terms of suppressing peeling of the measurement electrode 44 .

<Manufacturing process of sensor element>
Next, a process for manufacturing the sensor element 101 having the configuration and characteristics described above will be described. In the present embodiment, sensor element 101 is manufactured by forming a laminate of green sheets (also referred to as base tape) containing zirconia as a ceramic component, cutting and firing the laminate.

In the following, the case of manufacturing the sensor element 101 composed of six layers shown in FIG. 1 will be described as an example. In this case, six sheets corresponding to the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 are provided. A green sheet will be provided. FIG. 5 is a diagram showing the flow of processing when fabricating the sensor element 101. As shown in FIG.

When manufacturing the sensor element 101, first, a blank sheet (not shown), which is a green sheet on which no pattern is formed, is prepared (step S1). In the case of manufacturing the sensor element 101 consisting of six layers, six blank sheets are prepared corresponding to each layer.

The blank sheet is provided with a plurality of sheet holes used for positioning during printing and lamination. Such sheet holes are formed in advance by punching processing using a punching device or the like at the blank sheet stage prior to pattern formation. If the corresponding layer is a green sheet forming an internal space, the through portion corresponding to the internal space is also provided in advance by a similar punching process or the like. Also, the thickness of each blank sheet corresponding to each layer of the sensor element 101 does not need to be the same.

When the blank sheets corresponding to each layer are prepared, each blank sheet is subjected to pattern printing and drying processing (step S2). Specifically, various electrode patterns, heater element 72 and heater insulating layer 74 patterns, internal wiring patterns (not shown), and the like are formed.

At the timing of the pattern printing, the sublimation material for forming the first diffusion rate-controlling portion 11, the second diffusion rate-controlling portion 13, and the third diffusion rate-controlling portion 30 is also applied or arranged.

Each pattern is printed by applying a pattern forming paste prepared according to the characteristics required for each pattern to a blank sheet using a known screen printing technique. A known drying means can also be used for the drying process after printing.

However, among the formation of these patterns, a method different from the conventional method is adopted for the formation of the pattern that will eventually become the measurement electrode 44 . This point will be described later.

After the pattern printing for each blank sheet is completed, printing and drying processing of an adhesive paste for laminating and bonding the green sheets corresponding to each layer are carried out (step S3). A known screen printing technique can be used for printing the adhesive paste, and a known drying means can be used for drying after printing.

Subsequently, the green sheets coated with the adhesive are stacked in a predetermined order and pressed under predetermined temperature and pressure conditions to form one laminate (step S4). Specifically, green sheets to be laminated are stacked and held in a predetermined lamination jig (not shown) while positioning them by sheet holes, and the lamination jig is heated and pressurized by a lamination machine such as a known hydraulic press. by The pressure, temperature, and time for heating and pressurizing depend on the lamination machine to be used, but appropriate conditions may be determined so as to achieve good lamination.

After the laminate is obtained as described above, a plurality of portions of the laminate are cut to cut out individual units (referred to as element bodies) of the sensor elements 101 (step S5).

The cut element bodies are fired at a firing temperature of about 1300° C. to 1500° C. (step S6). Thereby, the sensor element 101 is produced. That is, the sensor element 101 is produced by integrally firing the solid electrolyte layer and the electrodes. The firing temperature at that time is preferably 1200° C. or higher and 1500° C. or lower (for example, 1400° C.). By integrally sintering in such a manner, each electrode in the sensor element 101 has sufficient adhesion strength.

The sensor element 101 thus obtained is housed in a predetermined housing and incorporated into the main body (not shown) of the gas sensor 100 .

<Method for forming measurement electrodes>
As described above, in the present embodiment, the sensor element 101 is provided with the measurement electrode 44 having the two-layer structure as described above. Formation of the measurement electrode 44 will be described.

FIG. 6 is a diagram showing the procedure for forming a pattern that will eventually become the measurement electrode 44. As shown in FIG. There are two methods, manufacturing method A and manufacturing method B, for forming the pattern that will be the measurement electrode 44 .

In the manufacturing method A, first, an electrolyte paste containing zirconia as a ceramic component like the green sheet is applied to the formation target position of the measurement electrode 44 on the upper surface of the green sheet for forming the first solid electrolyte layer 4 (step S21A). . The electrolyte paste is a paste obtained by mixing zirconia powder, which is a solid electrolyte, with a binder and other organic components.

Subsequently, a three-component mixture paste is applied in a superimposed manner on the coating film formed by applying the electrolyte paste (step S22). Here, the three-component mixed paste is a noble metal powder containing Pt as a main component, a zirconia powder as a ceramic component, and a sublimable material for forming pores CV in the finally formed measuring electrode 44 . A powder of a certain pore-forming material is mixed with a binder and other organic components to form a paste.

After the application of the three-component mixed paste is completed, the paste coating film (paste double film) in which the electrolyte paste and the three-component mixed paste are superimposed is pressed by a predetermined pressing means (step S23). ). By such pressing, part of the noble metal particles and the pore-forming material particles contained in the coating film of the three-component mixture paste penetrates into the coating film of the electrolyte paste.

After that, when the element body is fired as described above, the paste double film is also fired, and volatilization of the organic component and further sintering of the noble metal NM and the solid electrolyte SE proceed. At this time, the solid electrolyte in the electrolyte paste is integrated with the solid electrolyte in the green sheet, and the portion where the noble metal particles enter the electrolyte paste becomes the lower layer 44L as sintering progresses. On the other hand, in the portion where the three-component mixed paste is applied, the pore-forming material is sublimated as the noble metal NM and the solid electrolyte SE are sintered, forming pores CV. Finally, the noble metal NM, the solid electrolyte SE, and the pores CV are formed. A mixed upper layer 44U is obtained. As a result, a two-layered measuring electrode 44 as shown in FIG. 2 is formed.

Note that the coating thickness and coating area of the electrolyte paste and the three-component mixed paste are determined by taking into consideration the shrinkage due to firing and the entry of noble metal particles and pore-forming material particles due to pressing. The thicknesses of 44U and lower layer 44L and the planar area of measurement electrode 44 may be set to desired values. For example, the relationship between the coating thickness and coating area of the electrolyte paste and the three-component mixed paste, the thickness of the upper layer 44U and the lower layer 44L in the measurement electrode 44, and the planar area of the measurement electrode 44 is experimentally determined in advance. You may make it specify. Further, the mixing ratio of the noble metal powder, the solid electrolyte zirconia powder, and the pore-forming material powder in the three-component mixed paste is finally The volume ratio of noble metal NM, solid electrolyte SE, and pores CV in upper layer 44U (three-phase region) of measurement electrode 44 to be formed may be set to a desired value.

On the other hand, in manufacturing method B, the two-component mixed paste is applied to the formation target positions of the measurement electrodes 44 on the upper surface of the green sheet for forming the first solid electrolyte layer 4 (step S21B). Here, the two-component mixed paste is a paste obtained by mixing a noble metal powder containing Pt as a main component, a zirconia powder as a ceramic component, and a binder and other organic components.

After that, as in the manufacturing method A, the three-component mixed paste is applied in a superimposed manner on the coating film formed by applying the two-component mixed paste (step S22), and the two-component mixed paste and the three-component mixed paste are applied. is pressed by a predetermined pressing means (step S23).

Also in the case of manufacturing method B, part of the noble metal particles and pore-forming material particles contained in the coating film of the three-component mixed paste penetrates into the coated film of the two-component mixed paste by pressing.

After that, when the element body is fired as described above, the paste double film is also fired, and volatilization of the organic component and further sintering of the noble metal NM and the solid electrolyte SE proceed. As a result, the portion to which the two-component mixed paste is applied becomes the lower layer. On the other hand, in the portion where the three-component mixed paste is applied, the pore-forming material is sublimated as the noble metal NM and the solid electrolyte SE are sintered, forming pores CV. Finally, the noble metal NM, the solid electrolyte SE, and the pores CV are formed. A mixed upper layer 44U is obtained. Also in this case, as a result, a two-layered measuring electrode 44 as shown in FIG. 2 is formed.

Also in the case of manufacturing method B, the coating thickness and coating area of the two-component mixed paste and the three-component mixed paste are finally formed while considering shrinkage due to firing and penetration of noble metal particles and pore-forming material particles due to pressing. The thickness of the upper layer 44U and the lower layer 44L in the measurement electrode 44 and the planar area of the measurement electrode 44 may be set to desired values. In addition, the mixing ratio of the noble metal powder in the two-component mixed paste and the zirconia powder as the solid electrolyte is determined by taking into account the penetration of the noble metal particles and the pore-forming material particles by pressing, and the final measurement electrode 44 is formed. The volume ratio between the noble metal NM and the solid electrolyte SE in the lower layer 44L (two-phase region) may be set to a desired value. Similarly, the mixing ratio of the noble metal powder, the solid electrolyte zirconia powder, and the pore-forming material powder in the three-component mixed paste is determined by considering the penetration of the noble metal particles and the pore-forming material particles by pressing. The volume ratio of the noble metal NM, the solid electrolyte SE, and the pores CV in the upper layer 44U (three-phase region) of the measurement electrode 44 formed in 1 may be set to a desired value. As with process A, the relationship between these values may also be determined experimentally in advance.

As described above, according to the present embodiment, although the measuring electrode provided in the sensor element of the limiting current type gas sensor is provided as a porous cermet electrode as a whole, only the noble metal and the solid electrolyte are present and the pores are small. A two-layer structure consisting of a lower layer in which no metal is present and an upper layer in which noble metals, solid electrolytes, and pores are mixed to increase the degree of region boundary variation between the region made of the solid electrolyte and the region occupied by the noble metal and the pores. As a result, the deterioration of the measuring electrode due to the presence of surplus oxygen that cannot be taken into the measuring electrode is preferably suppressed. This realizes a gas sensor in which the deterioration of the measurement sensitivity is suitably suppressed even if it is used continuously.

<Modification>
A two-layer structure of a pore-free region and a pore-existing region, which is employed in the measurement electrode 44 in the above-described embodiment, is provided, and a region boundary between the region made of the solid electrolyte and the region occupied by the noble metal and the pores. A configuration in which the degree of variation is increased may be applied to the inner pump electrode 22 and the auxiliary pump electrode 51 which are provided as a cermet of Au—Pt alloy and ZrO 2 like the measurement electrode 44 . In such a case, the effect of suppressing peeling due to the anchor effect can be obtained even in these electrodes. Such inner pump electrode 22 and auxiliary pump electrode 51 can also be formed in the same process as the measurement electrode 44 .

In the above-described embodiment, two methods, manufacturing method A and manufacturing method B, are shown as the method of forming the pattern that becomes the measurement electrode 44, but manufacturing method C, which is an alternative forming method, may be adopted.

In manufacturing method C, the first solid electrolyte layer 4 is formed by using the electrolyte paste used in manufacturing method A, the two-component mixed paste used in manufacturing method B, and the three-component mixed paste used in manufacturing methods A and B in this order. After coating the target position of the measurement electrode 44 on the upper surface of the green sheet for the measurement, it is pressed by a predetermined pressing means.

Also in the case of manufacturing method C, the coating thickness and coating area of the electrolyte paste, the two-component mixed paste, and the three-component mixed paste are determined in consideration of shrinkage due to firing and penetration of noble metal particles and pore-forming material particles due to pressing. The thickness of the upper layer 44U and the lower layer 44L in the measurement electrode 44 to be formed, and the planar area of the measurement electrode 44 may be set to have desired values. In addition, the mixing ratio of the noble metal powder in the two-component mixed paste and the zirconia powder as the solid electrolyte is determined by taking into account the penetration of the noble metal particles and the pore-forming material particles by pressing, and the final measurement electrode 44 is formed. The volume ratio between the noble metal NM and the solid electrolyte SE in the lower layer 44L (two-phase region) may be set to a desired value. Similarly, the mixing ratio of the noble metal powder, the solid electrolyte zirconia powder, and the pore-forming material powder in the three-component mixed paste is determined by considering the penetration of the noble metal particles and the pore-forming material particles by pressing. The volume ratio of the noble metal NM, the solid electrolyte SE, and the pores CV in the upper layer 44U (three-phase region) of the measurement electrode 44 formed in 1 may be set to a desired value. As with process A and process B, the relationship between these values may also be determined experimentally in advance.

As an example, six types of gas sensors 100 (Examples 1 to 6) with different fabrication conditions for the measurement electrode 44 were fabricated. The volume ratio of each phase (noble metal, solid electrolyte, pores) and the degree of domain boundary variation were evaluated. In order to evaluate the degree of deterioration of the measuring electrodes due to continuous use, a continuous driving test was conducted in the air.

When forming the measurement electrode 44, the manufacturing method is divided into two levels, a manufacturing method A and a manufacturing method B. For each manufacturing method, the noble metal powder, the solid electrolyte powder, and the powder in the three-component mixed paste for forming the upper layer 44U are mainly used. The weight ratio of the pore material was changed to two levels. One type of electrolyte paste used in manufacturing method A and one type of two-component mixture paste used in manufacturing method B were used.

Further, as a conventional example, a gas sensor including a measurement electrode 44Z entirely formed of a three-component mixed paste was also manufactured (hereinafter, such a manufacturing method is referred to as a conventional manufacturing method). We evaluated the volume ratio of the phases (noble metal, solid electrolyte, and pores) and conducted a continuous drive test in the air.

Table 1 lists the weight ratios (component ratios) of the noble metal powder, the solid electrolyte powder, and the pore-forming material in the paste used to prepare the measuring electrode 44 of Examples 1 to 6 and the conventional measuring electrode 44Z. is shown.

In Table 1, the column "for upper layer formation" shows the weight ratio of the three-component mixed paste, and the column "for lower layer formation" shows the weight ratio of the electrolyte paste and the two-component mixed paste. there is

As shown in Table 1, the weight ratios of the noble metal powder, the solid electrolyte powder, and the pore-forming material used in the three-component mixed paste are a, b, and c, respectively, and the weights of the noble metal powder and the solid electrolyte powder used in the two-component mixed paste. When the ratios are d and e, respectively, for the three-component mixed paste, a:b:c=10:2:1 (Example 1, Example 4, Example 6) and a:b:c=30 : 10:1 (Example 2, Example 3, Example 5). In addition, regarding the two-component mixed paste, the value when the mixing ratio c of the pore-forming material in the three-component mixed paste was 1 was set to (c:) d:e = (1:) 5:1 ( Example 4, Example 5, Example 6). Furthermore, for the solid electrolyte powder contained in the electrolyte paste, c:e=1:10, where the ratio of the pore-forming material in the three-component mixed paste to the mixing ratio c of the pore-forming material in the three-component mixed paste was represented by e for convenience sake. Example 1, Example 2).

In Examples 1 to 3 in which the measurement electrode 44 was formed by the manufacturing method A, the target coating thickness of the electrolyte paste was set to 10 μm, and the target coating thickness of the three-component mixed paste was set to 15 μm.

In Examples 4 to 6 in which the measurement electrode 44 was formed by manufacturing method B, the target coating thickness of the two-component mixed paste was set to 5 μm, and the target coating thickness of the three-component mixed paste was set to 15 μm.

In the conventional example in which the measuring electrode 44Z was produced by the conventional manufacturing method, the three-component mixed paste had a:b:c=10:2:1, and the target coating thickness of the three-component mixed paste was set to 15 μm.

Table 2 shows the volume ratio of each phase in the upper layer 44U and the lower layer 44L of the measurement electrode 44 and the evaluation results of the region boundary variation degree for each example and conventional example. For convenience, the volume ratio of the measurement electrode 44Z of the conventional example is shown in the "upper layer" column.

In Table 2, the volume ratios of the noble metal NMs, the solid electrolyte SE, and the pores CV in the upper layer 44U are P1 (%), P2 (%), and P3 (%), respectively, and the noble metal NMs and solid electrolytes in the lower layer 44L The volume ratios of SE are P4 (%) and P5 (%), respectively. P1+P2+P3=P4+P5=100(%).

The volume ratio of each phase in the measuring electrode 44 and the measuring electrode 44Z was obtained by capturing a cross-sectional SEM image of each, and performing known image processing on the cross-sectional SEM image. Schematically, the cross-sectional SEM image is ternarized so that the region where the noble metal NM exists is white, the region where the solid electrolyte SE exists is gray, and the region where pores or internal voids exist is black. The ratio (cross-sectional area ratio) was taken as the volume ratio. In addition, the evaluation of the degree of region boundary variation is performed by using an SEM image of a cross section perpendicular to the thickness direction of the sensor element, including the first solid electrolyte layer 4 and the measurement electrode 44, to determine the difference between the first region and the second region. The distance between the boundary and the lowest end of the existence range of the noble metal NM was measured at intervals of 0.1 μm in the longitudinal direction of the element, and the average value and standard deviation σ thereof were obtained. The value of the latter standard deviation σ corresponds to the area boundary dispersion degree. However, for the conventional example, both the average value and the standard deviation σ are set to 0.

The results shown in Table 2 indicate that the measurement electrode 44 has a two-layer configuration of an upper layer 44U and a lower layer 44L in any of Examples 1 to 6.

In more detail, in Example 1, Example 4, and Example 6, in which the weight ratio a:b:c in the three-component mixed paste is 10:2:1, although the method of manufacturing the measurement electrode 44 is different, The values of P1, P2, P3, P4 and P5 were each the same. Of these, the volume ratio of the noble metal NM in the upper layer 44U and the lower layer 44L was P1=P4=40%. Regarding the volume ratio of the solid electrolyte SE, the upper layer 44U has a value P2 of 20%, while the lower layer 44L has a larger value P5 of 60%.

On the other hand, in Examples 2, 3, and 5, in which the weight ratio a:b:c in the three-component mixed paste is 30:10:1, although the manufacturing method of the measurement electrode 44 is different, P1, The values of P2 and P3 were the same, but the values of P4 and P5 differed by 5% between Examples 2 and 3 and Example 5. That is, in any of Examples 2, 3, and 5, the value of the volume ratio P1 of the noble metal NM in the upper layer 44U is 45%, and the value of the volume ratio P2 of the solid electrolyte SE is 40%. In contrast, the value of the volume ratio P4 of the noble metal NM in the lower layer 44L was 45% in Examples 2 and 3, and 40% in Example 5. Correspondingly, the volume ratio P5 of the solid electrolyte SE was 55% in Examples 2 and 3, and 60% in Example 5.

However, in any of the examples, the volume ratio of the solid electrolyte SE in the lower layer 44L was higher than the volume ratio of the solid electrolyte SE in the upper layer 44U. Specifically, the volume ratio of the solid electrolyte SE in the upper layer 44U was in the range of 20% to 40%, and the volume ratio of the solid electrolyte SE in the lower layer 44L was in the range of 50% to 60%. .

The volume ratios P1, P2, and P3 of each phase in the measurement electrode 44Z of the conventional example were the same as those of Examples 1 and 4 using the three-component mixed paste of the same weight ratio.

Further, the value of the standard deviation σ corresponding to the degree of region boundary variation was larger in Examples 4 to 6 produced by the production method B than in Examples 1 to 3 produced by the production method A. However, there is a difference in the value of the standard deviation σ between Examples (Examples 2 and 3, Examples 4 and 6) in which the manufacturing method was common and the weight ratio in the three-component mixed paste was the same. , and the corresponding relationship between the weight ratio and the value of the standard deviation σ varied depending on the manufacturing method.

In the continuous driving test in the atmosphere, each gas sensor was operated continuously for 3000 hours in an atmosphere of 25±5°C, and after the start of operation, after 1000 hours, after 2000 hours, and after 3000 hours, This was done by measuring the value of NOx current (also called NOx output) at the time of . The rate of change in NOx current at each measurement timing was calculated as the rate of change in NOx output based on the NOx current value at the start of operation. The NOx output change rate is a value that serves as an index indicating the degree of deterioration of the measurement electrode 44 or the measurement electrode 44Z.

FIG. 7 is a graph showing the results of the continuous drive test. In FIG. 7, the horizontal axis represents the drive time, and the vertical axis represents the NOx output change rate.

As shown in FIG. 7, in the gas sensor of the conventional example, the NOx output changes remarkably with the lapse of driving time, and the NOx output change rate after 3000 hours is about 11.5%±3.5% considering variations. In contrast, in the case of the gas sensors of Examples 1 to 6, the rate of change in NOx output remained at a maximum of about 6% even after 3000 hours had elapsed. This means that in the gas sensors of Examples 1 to 6, deterioration of the measurement electrodes is suppressed even after 3000 hours have passed.

In particular, in the gas sensors other than Example 3, the rate of change in the NOx output after 3000 hours remained generally at 3% or less, and in these gas sensors, the deterioration of the measurement electrodes was more preferably suppressed. be judged.

The results of the continuous driving test described above show that in the sensor element of the limiting current type gas sensor, the measurement electrode is arranged in the lower layer of the two-phase structure of the noble metal and the solid electrolyte and the upper layer of the three-phase structure of the noble metal, the solid electrolyte, and the pores. It is shown that deterioration of the measurement electrode can be suppressed when the gas sensor is used continuously by providing a two-layer structure with a layer and increasing the degree of area boundary variation.

1 to 3 First to Third Substrate Layers 4 First Solid Electrolyte Layer 5 Spacer Layer 6 Second Solid Electrolyte Layer 10 Gas Inlet Port 11 First Diffusion Rate Control Section 12 Buffer Space 13 Second Diffusion Rate Control Section 20 First Internal Space 21 main pump cell 22 inner pump electrode 23 outer pump electrode 30 third diffusion barrier 40 second internal cavity 41 measurement pump cell 42 reference electrode 44 measurement electrode 44L lower layer (of measurement electrode) 44U upper layer (of measurement electrode) 50 Auxiliary pump cell 51 Auxiliary pump electrode 60 Fourth diffusion control section 61 Third internal cavity 70 Heater section 80 Main sensor cell 81 Auxiliary sensor cell 82 Measurement sensor cell 100 Gas sensor 101 Sensor element CV Pore Ip0 Main pump current Ip1 Auxiliary pump current Ip2 NOx current NM Noble metal SE solid electrolyte

Claims (6)

A sensor element of a limiting current type NOx sensor,
a base portion having an oxygen ion-conducting solid electrolyte as a constituent material;
at least one internal cavity into which the gas to be measured is introduced;
an internal electrode arranged facing the at least one internal cavity; an external pump electrode arranged outside the at least one internal cavity; and the internal electrode of the base portion and the at least one pump cell consisting of: a portion existing between the outer cavity pump electrode;
with
the internal electrode comprises a noble metal, the solid electrolyte, and pores;
In the internal electrode, the degree of area boundary variation, which is the degree of variation in the element thickness direction, of the boundary between the base portion or the first region made of the solid electrolyte continuous from the base portion and the second region occupied by the noble metal and pores. is 0.5 μm or more and 6.5 μm or less,
A sensor element of a NOx sensor, characterized by: A sensor element of the NOx sensor according to claim 1,
the at least one internal space is a measurement internal space into which the gas to be measured whose oxygen concentration has been adjusted in advance is introduced;
wherein the internal electrode is a measuring electrode arranged in the internal measuring cavity;
wherein said at least one pump cell comprises said measuring electrode, said outer-cavity pumping electrode, and a portion of said base portion existing between said measuring electrode and said outer-cavity pumping electrode. be,
A sensor element of a NOx sensor, characterized by: A sensor element of the NOx sensor according to claim 2,
The measurement electrode is
an upper layer comprising the noble metal, the solid electrolyte and pores;
a lower layer comprising the noble metal and the solid electrolyte;
with
The volume ratio of the solid electrolyte in the upper layer is 20% to 40%,
The volume ratio of the solid electrolyte in the lower layer is 50% to 60%,
A sensor element of a NOx sensor, characterized by: A sensor element of the NOx sensor according to claim 3,
the thickness ratio of the upper layer to the lower layer is in the range of 95:5 to 10:90;
A sensor element of a NOx sensor, characterized by: A sensor element of the NOx sensor according to claim 4,
the thickness ratio of the upper layer to the lower layer is in the range of 90:10 to 30:70;
A sensor element of a NOx sensor, characterized by: A sensor element of the NOx sensor according to any one of claims 1 to 5,
The area boundary variation degree is 1 μm or more and 5 μm or less,
A sensor element of a NOx sensor, characterized by:

Images