JP2021051061A - Sensor element, gas sensor, and gas sensor unit - Google Patents

Abstract

To provide a sensor element, gas sensor, and gas sensor unit that prevent an electrode from peeling from a sold electrolyte body while improving the oxygen decomposition activity of the electrode in a pump cell.SOLUTION: In a sensor element comprising a pump cell for adjusting the oxygen concentration including a measurement chamber, a sold electrolyte body, and an inner electrode exposed to the measurement chamber, and an outer electrode arranged outside of the measurement chamber, and a reference cell for generating voltage corresponding to the oxygen concentration in the measurement chamber, at least one electrode, composed of a noble metal and a component of the solid electrolyte body, has a noble metal region 205, a solid electrolyte body region 203, and a coexistence region 207 in which the noble metal and the component of the solid electrolyte body coexist when a cross section is observed along the thickness direction, and an area ratio SR of the coexistence region represented by {coexistence region/(noble metal region + solid electrolyte body region + coexistence region)} in a cross section of at least one electrode is between 15.5% to less than 30%.SELECTED DRAWING: Figure 4

Description

The present invention relates to a sensor element, a gas sensor and a gas sensor unit.

With the tightening of regulations on exhaust gas emitted from internal combustion engines such as automobiles, it is required to reduce the amount of nitrogen oxides (NOx) in the exhaust gas. Therefore, in recent years, the development of NOx sensors capable of directly measuring the NOx concentration in exhaust gas has been progressing. The NOx sensor includes a NOx sensor element having a pump cell formed by forming a pair of electrodes on the surface of an oxygen ion conductive solid electrolyte such as zirconia and a NOx concentration detecting cell.

The NOx sensor pumps or pumps oxygen in the measurement chamber communicating with the space to be measured containing NOx by a pump cell. At this time, the pump cell is controlled so that the measurement chamber has a predetermined oxygen concentration. Further, the NOx concentration of the gas to be measured whose oxygen concentration is controlled (adjusted) is detected in the detection cell.
In the NOx sensor having the above configuration, the electrodes of the NOx sensor element (detection element) are not sufficiently activated and sufficient sensor characteristics cannot be obtained only by providing the electrodes on the solid electrolyte.
Therefore, a technique has been proposed in which a voltage is applied between a pair of electrodes of a pump cell and an aging treatment is performed to improve the oxygen decomposition activity (see Patent Document 1). In this technology, it is said that the oxygen decomposition activity is improved by setting the ratio of the mixed region containing the noble metal contained in the electrode and the component of the solid electrolyte to 30% or more in the cross section along the thickness direction of the electrode. ..

Japanese Patent No. 6382162

However, since the mixed region is inferior in bondability, there is a problem that if the mixed region is too large, the electrode is likely to be separated from the solid electrolyte. Further, when the current flowing through the pump cell is feedback-controlled so that the potential of the reference cell becomes constant, the difference in responsiveness between the electrode of the pump cell including the mixed region and the electrode of the reference cell becomes large, which may cause oscillation. Can be.
On the other hand, if the mixed region is too small, the internal resistance of the electrode rises at a low temperature, the voltage of the pump cell (Vp1) becomes high, and the component to be measured in the exhaust gas is decomposed.
The present invention has been made in view of this background, and an object of the present invention is to provide a sensor element, a gas sensor, and a gas sensor unit that improve the oxygen decomposition activity of an electrode in a pump cell and suppress the electrode from peeling from a solid electrolyte. And.

In order to solve the above problems, the sensor element of the present invention includes a measurement chamber, a solid electrolyte, an inner electrode formed on the surface of the solid electrolyte and exposed to the measurement chamber, and a surface of the solid electrolyte. It has an outer electrode formed in the measurement chamber and arranged outside the measurement chamber, and the oxygen concentration in the measurement chamber is increased by pumping and pumping oxygen in the gas to be measured introduced into the measurement chamber. A sensor element having a pump cell to be adjusted and a reference cell that generates a voltage corresponding to the oxygen concentration in the gas to be measured in the measurement chamber, wherein at least one of the inner electrode and the outer electrode is , The noble metal region composed of the noble metal and the solid electrolyte region composed of the components of the solid electrolyte body when the noble metal and the component of the solid electrolyte body are contained and the cross section is observed along the thickness direction. , The coexistence region in which the noble metal and the component of the solid electrolyte coexist, and in the cross section of the at least one electrode, {the coexistence region / (the noble metal region + the solid electrolyte region + the said Coexistence region)} is characterized in that the area ratio SR of the coexistence region is 15.5% or more and less than 30%.

According to this sensor element, the oxygen decomposition activity of the above-mentioned one electrode in the pump cell can be improved, and since there are not too many coexisting regions having poor binding properties, it is possible to prevent the above-mentioned one electrode from peeling off from the solid electrolyte. ..
Further, when the current flowing through the pump cell is feedback-controlled so that the potential of the reference cell becomes constant, the difference in responsiveness between the above-mentioned one electrode including the coexistence region and the electrode of the reference cell does not become too large. Oscillation can be suppressed.
Further, it is possible to suppress an increase in the internal resistance of one of the electrodes described above due to too few coexisting regions, and it is also possible to suppress an increase in the voltage (Vp1) of the pump cell and decomposition of the gas component to be measured. Vp1 is a voltage (pump voltage) applied between both electrodes of the pump cell when a pump current (Ip1) is passed between both electrodes of the pump cell in a positive direction or a negative direction to pump or pump oxygen. ..

The sensor element of the present invention further has a NOx detection cell for measuring the concentration of nitrogen oxides in the gas to be measured after adjusting the oxygen concentration, and at least one of the electrodes has at least the inner electrode. Including, the area ratio SR of the coexisting region in the cross section of the inner electrode may be 15.5% or more and less than 30%.
According to this sensor element, in a gas sensor having a NOx detection cell in general, the adjustment of the oxygen concentration is almost the pumping out of oxygen, and the inner electrode has the coexistence region of the above ratio, so that the oxygen concentration can be adjusted particularly effectively. Can be done.

In the sensor element of the present invention, the area ratio SR of the coexisting region may be 16% or more and 27% or less.

The gas sensor of the present invention includes the sensor element and a main metal fitting for holding the sensor element.

The gas sensor unit of the present invention includes the gas sensor and a gas sensor control unit connected to the gas sensor, and the gas sensor control unit feedback-controls the current flowing through the pump cell so that the potential of the reference cell becomes constant. It is characterized by doing.

According to the present invention, it is possible to improve the oxygen decomposition activity of the electrode in the pump cell and suppress the electrode from peeling from the solid electrolyte.

It is sectional drawing which follows the axial direction of a NOx sensor. It is a perspective view which shows the sensor element by omitting a part in the axial direction. It is explanatory drawing which shows the tip side of a sensor element by breaking in the thickness direction, and its internal structure is enlarged. It is a figure which shows the reflected electron image obtained by FE-SEM of the cross section along the thickness direction of the 1st electrode. It is a figure which shows the image which made the composition (configuration) of the image of FIG. 4 based on the size and the degree of dispersion of the gray scale value of each dot. It is a figure which shows the structure of the sensor control device. It is a figure which shows the relationship between the temperature of a sensor element, and Vp1 when the area ratio of a coexistence region is changed. It is a figure which shows Ip1 (more precisely, a noise level) when the area ratio of a coexistence region is changed.

Hereinafter, the gas sensor (NOx sensor) 1 according to the embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view taken along the axis O direction of the NOx sensor 1. In the following, the lower side in FIG. 1 is referred to as the front end side of the NOx sensor 1, and the upper side in FIG. 1 is referred to as the rear end side of the NOx sensor 1.

As shown in FIG. 1, the NOx sensor 1 includes a main metal fitting 5, a sensor element (NOx sensor element) 7, a ceramic sleeve 9, an insulating separator 13, and six lead frames (terminal metal fittings) 15. Be prepared. Note that FIG. 1 shows only a part of the six lead frames 15.
The sensor element 7 is a plate-shaped laminated member extending in the axis O direction. The sensor element 7 penetrates the main metal fitting 5 and the tip end side is directed to the exhaust gas to be measured. The exhaust gas corresponds to the gas to be measured. A detection unit 17 is formed on the tip end side of the sensor element 7, and the detection unit 17 is covered with a protective layer (not shown).

As shown in FIG. 2, electrode pads 23, 25, 27, 29, 31, 33 are formed on the rear end side of the sensor element 7. The electrode pads 23, 25, and 27 are formed on the first plate surface 19 which is one surface of the outer surface of the sensor element 7. The electrode pads 29, 31, and 33 are formed on the second plate surface 21 of the outer surface of the sensor element 7, which is the surface opposite to the first plate surface 19.

The ceramic sleeve 9 has a tubular shape and is arranged so as to surround the radial circumference of the sensor element 7. The insulating separator 13 is made of, for example, an insulating material made of alumina and has an element insertion hole 11 penetrating in the axis O direction. The element insertion hole 11 surrounds at least a part of the sensor element 7 and the lead frame 15.
The insulating separator 13 holds the lead frame 15 and the sensor element 7 inside the element insertion hole 11, so that the lead frame 15 is electrically connected to the electrode pads 23 to 33 of the sensor element 7, respectively. Further, the lead frame 15 is also electrically connected to the lead wire 35 disposed inside the sensor from the outside, and flows between the external device to which the lead wire 35 is connected and the electrode pads 23 to 33. Form a current path for current.

The main metal fitting 5 is, for example, a metal member having a substantially tubular shape made of stainless steel. The main metal fitting 5 has a through hole 37 penetrating in the axis O direction, and has a shelf portion 39 protruding inward in the radial direction inside the through hole 37. A screw portion 3 for fixing to the exhaust pipe is formed on the outer surface of the main metal fitting 5. The main metal fitting 5 is configured to hold the sensor element 7 inserted through the through hole 37, and the detection unit 17 of the sensor element 7 is arranged outside the tip side of the through hole 37 and outside the rear end side of the through hole 37. It is a state in which the electrode pads 23 to 33 are arranged.

Inside the through hole 37, an annular ceramic holder 41, a powder filling layer (talc ring) 43, 45, and the above-mentioned ceramic sleeve 9 are arranged in this order from the tip side in a state of surrounding the radial circumference of the sensor element 7. It is laminated toward the rear end side.
A crimping ring 49 is arranged between the ceramic sleeve 9 and the rear end portion 47 of the main metal fitting 5. A metal cup 51 is arranged between the ceramic holder 41 and the shelf portion 39 of the main metal fitting 5. The rear end portion 47 is crimped so as to press the ceramic sleeve 9 toward the tip side via the crimping ring 49.

On the tip end side of the main metal fitting 5, for example, a tubular protector 52 made of stainless steel is arranged so as to cover the tip end side of the sensor element 7. The protector 52 has a vent 53 through which exhaust gas can pass. The protector 52 has a double protector structure including an inner protector and an outer protector.
An outer cylinder 55 made of, for example, stainless steel is fixed to the rear end side of the main metal fitting 5. The opening 57 on the rear end side of the outer cylinder 55 is closed by, for example, a grommet 59 made of fluororubber.
The insulating separator 13 is held in the outer cylinder 55 with its rear end side in contact with the grommet 59. The insulating separator 13 is held by the holding member 61. The holding member 61 is fixed to the inside of the outer cylinder 55 by crimping.

Next, the configuration of the sensor element 7 will be described with reference to FIGS. 2 to 3. FIG. 2 is a perspective view showing the sensor element 7 with a part omitted in the axis O direction, and FIG. 3 is an explanatory view showing the tip side of the sensor element 7 cut in the thickness direction and the internal structure thereof is enlarged.
As shown in FIG. 2, the sensor element 7 has a plate shape having a rectangular shaft cross section, and has a structure in which the element portion 63 and the heater 65 are laminated. The element portion 63 and the heater 65 are each formed in a plate shape extending in the axis O direction.
As shown in FIG. 3, the sensor element 7 has an insulating layer 67, a first solid electrolyte 69, an insulating layer 71, a second solid electrolyte 73, an insulating layer 75, and a third solid electrolyte in this order from the top in FIG. It has a structure in which 77 and insulating layers 79 and 81 are laminated. Of these, the insulating layer 67, the first solid electrolyte 69, the insulating layer 71, the second solid electrolyte 73, the insulating layer 75, and the third solid electrolyte 77 correspond to the element unit 63. The sensor element 7 includes a first pump cell 83, a reference cell 85, and a second pump cell 87.

A first measurement chamber 89 is formed between the first solid electrolyte body 69 and the second solid electrolyte body 73. In FIG. 3, the left end of the first measurement chamber 89 is the entrance. A first diffusion resistance portion 91 is arranged at this inlet. The exhaust gas GM is introduced into the first measurement chamber 89 from the outside via the first diffusion resistance portion 91. A second diffusion resistance portion 93 is arranged at an end of the first measurement chamber 89 opposite to the inlet.
A second measurement chamber 95 is formed on the right side of the second diffusion resistance portion 93. The second measurement chamber 95 communicates with the first measurement chamber 89 via the second diffusion resistance portion 93. The second measurement chamber 95 is formed between the first solid electrolyte body 69 and the third solid electrolyte body 77. The portion of the second solid electrolyte body 73 corresponding to the second measurement chamber 95 is cut out.

The first to third solid electrolytes 69, 73, and 77 each contain zirconia having oxygen ion conductivity as a main component. Each of the insulating layers 67, 71, 75, 79, and 81 contains alumina as a main component. The first and second diffusion resistance portions 91 and 93 are each made of a porous substance such as alumina. The main component means a component having a content of 50% by mass or more in the ceramic layer.
A resistance heating element 97 is embedded between the insulating layers 79 and 81. The resistance heating element 97 extends along the left-right direction in FIG. The resistance heating element 97 is made of, for example, platinum. The insulating layers 79 and 81 and the resistance heating element 97 form a heater 65. The heater 65 raises the temperature of the sensor element 7 to a predetermined active temperature, enhances the conductivity of oxygen ions of the first to third solid electrolytes 69, 73, and 77, and exerts an action of stabilizing the operation.

The first pump cell 83 includes a first solid electrolyte body 69, a first electrode 101, and a second electrode 99. The first electrode 101 and the second electrode 99 sandwich the first solid electrolyte body 69.
The first electrode 101 includes platinum, zirconia, a coexistence region described later, and pores. Zirconia corresponds to the ceramic component contained in the first solid electrolyte 69. The second electrode 99 contains platinum as a main component.
The first electrode 101 faces the first measurement chamber 89. The surface of the first electrode 101 is covered with a porous layer 107 through which gas can pass. The second electrode 99 faces the outside of the sensor element 7. The second electrode 99 is covered with a porous layer 105. The porous layer 105 is embedded in the opening 103 of the insulating layer 67. The porous layer 105 is made of a porous body through which a gas such as oxygen can pass. Examples of this porous body include alumina and the like.

The reference cell 85 includes a second solid electrolyte body 73, a third electrode 109, and a fourth electrode 111. The reference cell 85 generates a voltage (based on the atmosphere of the gas to be measured in the first measurement chamber 89) according to the oxygen concentration in the gas to be measured in the first measurement chamber 89. The third electrode 109 and the fourth electrode 111 sandwich the second solid electrolyte body 73. The third electrode 109 faces the first measurement chamber 89. The fourth electrode 111 faces the reference oxygen chamber 113, which will be described later. The third electrode 109 and the fourth electrode 111 each contain platinum as a main component.
The reference oxygen chamber 113 is a portion of the insulating layer 75 cut out. The reference oxygen chamber 113 is a space surrounded by a second solid electrolyte body 73, a third solid electrolyte body 77, and an insulating layer 75. In the reference oxygen chamber 113, the oxygen concentration is maintained at a predetermined concentration.

The second pump cell 87 includes a third solid electrolyte body 77, a fifth electrode 115, and a sixth electrode 117. The fifth electrode 115 and the sixth electrode 117 are formed on one surface of the third solid electrolyte body 77, respectively. The fifth electrode 115 faces the second measurement chamber 95. The sixth electrode 117 faces the reference oxygen chamber 113. The fifth electrode 115 and the sixth electrode 117 are separated by an insulating layer 75. The fifth electrode 115 and the sixth electrode 117 each contain platinum as a main component. The sixth electrode 117 is covered with an insulating protective layer 165 made of porous material. In the reference oxygen chamber 113, there is a void 167 which is an unfilled space.

The first measurement chamber 89, the first solid electrolyte 69, the first electrode 101, the second electrode 99, and the first pump cell 83 are the "measurement chamber", the "solid electrolyte", and the "inside", which are within the scope of the claims, respectively. Corresponds to "electrode", "outer electrode", and "pump cell".
The reference cell 85 corresponds to the "reference cell" in the claims.
The second pump cell 87 corresponds to the “NOx detection cell” in the claims.
Further, the sensor control device 169 described later corresponds to the "gas sensor control unit" in the claims.

Next, the first electrode 101 will be described with reference to FIG. FIG. 4 is a reflected electron image obtained by FE-SEM having a cross section along the thickness direction of the first electrode 101.
As described above, when the cross section 201 is observed, the first electrode 101 has a noble metal region 205 made of platinum (precious metal), a solid electrolyte region 203 made of zirconia (a component of the solid electrolyte), and a noble metal and a solid. It includes a coexistence region 207 in which the components of the electrolyte coexist, and pores 209.
As the cross section 201, for example, an SEM image having a visual field size of 8.5 μm × 12 μm can be used.

The area ratio SR of the coexisting region represented by {coexistence region 207 / (precious metal region 205 + solid electrolyte region 203 + coexistence region 207)} is 15.5% or more and less than 30%.
As a result, the oxygen decomposition activity of the first electrode 101 in the first pump cell 83 can be improved, and since there are not too many coexisting regions having poor binding properties, it is possible to prevent the first electrode 101 from peeling off from the first solid electrolyte body 69. ..
Further, when the current flowing through the first pump cell 83 is feedback-controlled so that the potential of the reference cell 85 becomes constant, the responsiveness of the first electrode 101 including the coexistence region and the detection electrode (third electrode 109) Oscillation can be suppressed without the difference becoming too large.
Further, it is possible to suppress an increase in the internal resistance of the first electrode 101 due to too small a coexistence region, and it is also possible to suppress an increase in the voltage of the first pump cell 83 to decompose the gas component to be measured.

When the area ratio SR of the coexistence region 207 is less than 15.5%, the internal resistance of the first electrode 101 increases at a low temperature, the voltage (Vp1) of the first pump cell 83 increases, and the measurement target in the gas to be measured The specific gas, which is a component, is decomposed, and the measurement accuracy is lowered.
On the other hand, when the area ratio SR of the coexistence region 207 is 30% or more, the coexistence region 207 is inferior in the bondability at the time of electrode firing, so that the first electrode 101 is easily peeled off from the first solid electrolyte body 69. Further, when the current (Ip1) flowing through the first pump cell 83 is feedback-controlled so that the potential of the reference cell 85 becomes constant, the difference in responsiveness between the first electrode 101 including the coexistence region 207 and the third electrode 109 Becomes large and can cause oscillation.
The oscillation (noise) of Ip1 described above is due to feedback control of the first pump cell 83 based on the voltage of the reference cell 85.

The noble metal region 205, the solid electrolyte region 203, the coexistence region 207, and the pores 209 in the cross section 201 are discriminated as follows.
First, the cross section 201 is manufactured by FIB processing.
Then, elemental analysis is performed on this cross section 201 by STEM / EDS, and whether O (oxygen) is detected or not indicates whether it is a coexistence region 207 or an alloy. When O is detected, the component of the solid electrolyte exists in the state of ZrO2, so that it is the coexistence region 207. If O is not detected, the component of the solid electrolyte is present in the state of metal Zr, so it is an alloy.
Further, the noble metal region 205 and the solid electrolyte region 203 can be identified from the following EDS analysis.
Since the pores 209 have almost no response to the incident electrons in the SEM image, they appear black and can be distinguished from other regions.

Next, the area of the coexistence region 207 can be obtained from EDS analysis and SEM image analysis. Specifically, by EDS analysis, the area A of the region containing the noble metal (Pt) (= noble metal region 205 + coexistence region 207 + alloy) and the area B of the region containing Zr (= solid electrolyte region 203 + coexistence region 207 + alloy). And are required respectively.
(Area A + Area B) = solid electrolyte region 203 + noble metal region 205 + 2 × coexistence region 207 + 2 × alloy.
Next, the area C (= solid electrolyte region 203 + noble metal region 205 + coexistence region 207 + alloy) of the region excluding the pores 209 in the entire visual field is determined.
(Area A + Area B) -Area C = (Coexistence region 207 + Alloy). Excluding the area of the alloy (region where O is not detected) as described above, the area of the coexistence region 207 can be obtained.
That is, the coexistence region 207 is a portion excluding the region where Zr is detected in EDS and O is not detected, and the area ratio SR is 15.5%.

As shown in FIG. 5, the noble metal region 205, the solid electrolyte region 203, the coexistence region 207, and the pores 209 are discriminated by adjusting the contrast of the reflected electron image of the SEM to make the contrast of each region stand out. And the areas A, B, and C can be surely obtained.
Here, for example, the SEM image in BMP format is converted to CSV format, and the gray scale is converted into the numerical value (RGB: 0 to 255) of the color scale at that time. After that, it is possible to distinguish the noble metal region 205, the solid electrolyte region 203, the coexistence region 207, and the pores 209 by determining that the regions in which the RGB numerical range is within a predetermined range are the same region and performing grouping. ..
Both the noble metal and the solid electrolyte coexist in the coexistence region 207. For example, when the RGB dispersion degree of the noble metal and the solid electrolyte in the area of 10 × 10 dots is equal to or higher than a certain value, the coexistence region It can be distinguished by the judgment that it is.

As a method of controlling the area ratio SR of the coexistence region to 15.5% or more and less than 30%, it is possible to control conditions such as the temperature, applied voltage, and applied time of the aging treatment of the first electrode 101. For example, when the temperature and applied voltage of the aging treatment are increased and the applied time is lengthened, the area ratio SR of the coexisting region tends to increase.
The conditions of the aging process are as follows, for example.
Atmosphere of 1st electrode: Rich atmosphere Temperature of 1st electrode: 800 ° C or higher and lower than 950 ° C Voltage between 1st electrode and 2nd electrode: 0.75 to 1.00V
Aging process time: 40-200 sec
The rich atmosphere is an atmosphere in which the ratio of oxygen is small with respect to the theoretical empty fuel ratio (λ = 1). The theoretical empty fuel ratio is the mixing ratio of air and fuel that enables ideal complete combustion.
If the aging treatment temperature is too high, the internal resistance of the first solid electrolyte 69 decreases, and the current flowing through the first solid electrolyte 69 increases during the aging process. Since the aging treatment is usually performed in an oxygen-deficient atmosphere, there is a concern that if the aging treatment temperature is too high, a blackening phenomenon will occur in which the oxygen contained in the first solid electrolyte 69 is pumped in an attempt to pass an electric current. There is. Therefore, the aging temperature is preferably 1000 ° C. or lower.

Further, the area ratio SR of at least one coexisting region of the first electrode 101 and the second electrode 99 may be in the above range, and even if the area ratio SR of both coexisting regions of both electrodes 101 and 99 is in the above range. Good. In particular, when the area ratio SR of at least the coexisting region of the first electrode 101 is within the above range, the voltage fluctuation of the pump cell due to the flow velocity of the gas to be measured or the like is suppressed, and the measurement accuracy is improved.
For example, in the case of a NOx sensor, NOx is abundant at the time of leaning and moves in the direction of pumping out oxygen at the time of leaning, so that the effect of defining the area ratio SR of the first electrode 101 is high. Further, even in the case of an oxygen sensor, the effect of defining the area ratio SR of the first electrode 101 at the time of leaning is high.
The means for heating the first electrode 101 and / or the second electrode 99 in the aging treatment may be a heater 65 or an external heater.
The method of applying a positive voltage and a negative voltage between the first electrode and the second electrode may be an alternating voltage. First, the aging process of one electrode is completed with one of the polarities, and then the other with the opposite polarity. The electrode may be aged.

Next, the configuration of the sensor control device 169 that controls the operation of the sensor element 7 will be described with reference to FIG. FIG. 6 is a diagram showing the configuration of the sensor control device 169.
The sensor control device 169 includes a microcomputer 171 and an electric circuit unit 173. The microcomputer 171 includes a CPU 175 that executes various calculations, a RAM 177 that stores calculation results and the like, and a ROM 179 that stores programs and the like executed by the CPU 175.
Further, the microcomputer 171 includes an A / D converter 181 and a signal input / output unit 185, a timer clock (not shown), and the like. The signal input / output unit 185 is connected to the electric circuit unit 173 via the A / D converter 181 and communicates with the ECU 183.
The electric circuit unit 173 includes a reference voltage comparison circuit 187, an Ip1 drive circuit 189, a Vs detection circuit 191 and an Icp supply circuit 193, a resistance detection circuit 194, an Ip2 detection circuit 195, a Vp2 application circuit 197, and a heater drive circuit 199. To. The electric circuit unit 173 detects the NOx concentration in the exhaust gas GM by using the sensor element 7 under the control of the microcomputer 171.
The first electrode 101, the third electrode 109, and the fifth electrode 115 are connected to a reference potential. Further, one electrode of the resistance heating element 97 is grounded.

Next, the process of detecting the NOx concentration in the exhaust gas GM, which is executed by the sensor control device 169 and the NOx sensor 1, will be described. The heater drive circuit 199 passes a drive current through the resistance heating element 97. The resistance heating element 97 raises the temperature to heat and activate the first to third solid electrolytes 69, 73, 77. As a result, the first pump cell 83, the reference cell 85, and the second pump cell 87 come into operation.
The exhaust gas GM is introduced into the first measurement chamber 89 while being restricted by the flow amount of the first diffusion resistance unit 91. Here, the Icp supply circuit 193 causes a weak current Icp to flow from the fourth electrode 111 to the third electrode 109 in the reference cell 85. Therefore, oxygen in the exhaust gas GM can receive electrons from the third electrode 109 in the first measurement chamber 89 on the negative electrode side, becomes oxygen ions, flows in the second solid electrolyte body 73, and is a reference. Move into the oxygen chamber 113. That is, the oxygen in the first measurement chamber 89 is sent into the reference oxygen chamber 113 by the current Icp flowing between the third electrode 109 and the fourth electrode 111.

The Vs detection circuit 191 detects the voltage Vs between the third electrode 109 and the fourth electrode 111. The voltage Vs is a voltage corresponding to the difference in oxygen concentration between the inside of the first measurement chamber 89 and the inside of the reference oxygen chamber 113. The Vs detection circuit 191 compares the detected voltage Vs with the reference voltage (425 mV) using the reference voltage comparison circuit 187, and outputs the comparison result to the Ip1 drive circuit 189. Here, as the voltage Vs becomes constant in the vicinity of 425 mV, by adjusting the concentration of oxygen in the first measurement chamber 89, the oxygen concentration in the exhaust gas GM in the first measurement chamber 89 is a predetermined value (e.g., 10 ^- It will approach ⁸ to 10 ^{-8 atm).}

In the Ip1 drive circuit 189, when the oxygen concentration of the exhaust gas GM introduced in the first measurement chamber 89 is lower than a predetermined value, the current Ip1 is passed through the first pump cell 83 so that the second electrode 99 side becomes the negative electrode, and the sensor. Only oxygen is introduced into the first measurement chamber 89 from the outside of the element 7. On the other hand, when the oxygen concentration of the exhaust gas GM introduced into the first measurement chamber 89 is higher than a predetermined value, the Ip1 drive circuit 189 passes a current Ip1 through the first pump cell 83 so that the first electrode 101 side becomes the negative electrode. , Only oxygen is discharged from the inside of the first measurement chamber 89 to the outside of the sensor element 7.

The exhaust gas GM whose oxygen concentration has been adjusted in the first measurement chamber 89 is introduced into the second measurement chamber 95 via the second diffusion resistance portion 93. NOx in the exhaust gas GM in contact with the fifth electrode 115 in the second measurement chamber 95 is subjected to a fifth voltage Vp2 by applying a voltage Vp2 between the sixth electrode 117 and the fifth electrode 115 by the Vp2 application circuit 197. It is decomposed into _{N 2} and O ₂ on the electrode 115. The decomposed oxygen becomes oxygen ions, flows through the third solid electrolyte body 77, and moves into the reference oxygen chamber 113. Therefore, the current flowing through the second pump cell 87 shows a value corresponding to the NOx concentration.

The sensor control device 169 detects the current Ip2 flowing through the second pump cell 87 by the Ip2 detection circuit 195, and detects the NOx concentration in the exhaust gas GM from the current Ip2. Specifically, the relationship between the NOx concentration and the current Ip2 is obtained in advance, a map or the like is prepared in advance, and the measured current Ip2 is referred to this map to obtain the NOx concentration.

The NOx sensor 1 can be manufactured, for example, as follows.
First, a ceramic sheet as a raw material for the insulating layer 67, the first solid electrolyte 69, the second solid electrolyte 73, the third solid electrolyte 77, and the insulating layers 79 and 81 is prepared. Through holes and the like are appropriately formed in the ceramic sheet. In addition, the insulating layers 71 and 75 are formed by screen printing on the ceramic sheet.
Next, in order to form the electrodes 99, 101, 109, 111, 115, 117, a paste containing the raw materials of the electrodes is applied to the surface of the corresponding ceramic sheet. The paste for forming the first electrode 101 contains _{platinum and ZrO 2.} The paste for forming other electrodes contains platinum as a main component.

Next, the ceramic sheets are laminated to prepare a laminate, and the laminate is fired. At this time, the first electrode precursor is formed in the portion that will later become the first electrode 101. The first electrode precursor contains platinum and ZrO ₂ . When the mass of platinum contained in the first electrode precursor is 100 parts by mass, the _{mass of ZrO 2} contained in the first electrode precursor is 22 parts by mass. Further, in the first electrode precursor, the volume occupied by platinum is 56% by volume, and the _{volume occupied by ZrO 2} is 44% by volume.

Next, the first electrode precursor is subjected to an aging treatment. The conditions of the aging treatment are as described above. The temperature of the first electrode precursor is a value measured using an infrared radiation thermometer manufactured by Chino.
By the aging treatment, the first electrode 101 is formed from the first electrode precursor, and the gas sensor element 7 is completed.
The portion of the NOx sensor 1 other than the gas sensor element 7 can be manufactured by a known method.

It goes without saying that the present invention is not limited to the above-described embodiment, and can be implemented in various embodiments without departing from the present invention.

Of the first solid electrolyte body 69, all or part of the portion not sandwiched between the first electrode 101 and the second electrode 99 may be made of a material other than the solid electrolyte. Examples of materials other than the solid electrolyte include alumina and the like.
Further, in the second solid electrolyte body 73, all or a part of the portion not sandwiched between the third electrode 109 and the fourth electrode 111 may be made of a material other than the solid electrolyte. Examples of materials other than the solid electrolyte include alumina and the like.

The sensor of the present disclosure may be a sensor other than the NOx sensor. For example, it may be a sensor obtained by removing the second pump cell 87 from the NOx sensor 1 described above. This sensor can be a sensor that measures the oxygen concentration in the gas to be measured based on the energization amount Ip1.

The ceramic component contained in the first solid electrolyte body 69 and the first electrode 101 may be other than zirconia. Examples of ceramic components other than zirconia include CeO ₂ (ceria), ThO ₂ (tria), HfO ₂ , Bi ₂ O _{3, and the} like.
In the case of the ceramic component CeO ₂ , the aging treatment produces a coexistence region containing _{CeO 2 in the first electrode 101.} When the ceramic component is ThO ₂ , HfO ₂ , or Bi ₂ O ₃ , a coexistence region containing each of them is similarly generated.

The function of one component in each of the above embodiments may be shared by a plurality of components, or the function of the plurality of components may be exerted by one component. Further, a part of the configuration of each of the above embodiments may be omitted. In addition, at least a part of the configuration of each of the above embodiments may be added or replaced with respect to the configuration of the other embodiment. It should be noted that all aspects included in the technical idea specified from the wording described in the claims are embodiments of the present disclosure.
In addition to the NOx sensor described above, the present disclosure can be realized in various forms such as a system having the NOx sensor as a component.

[Example 1]
The NOx sensor 1 was manufactured by the method described above.
The temperature conditions for the aging treatment were as follows.
Atmosphere of 1st Electrode Precursor: Rich Atmosphere Temperature of 1st Electrode Precursor: 800-870 ° C
The temperature of the first electrode precursor is a value measured using an infrared radiation thermometer manufactured by Chino.

The conditions other than the aging process are as follows.
Atmosphere of 1st Electrode Precursor in Aging Treatment: H ₂ = 2.35% by Volume, H ₂ O = 0.8% by Volume, N ₂ = Between 1st Electrode Precursor and 2nd Electrode 99 in Remaining Aging Treatment Voltage: 0.77V
Aging process time: 40 sec
After the aging treatment, the area ratio SR of the coexisting region in the first electrode 101 of the NOx sensor 1 was measured by the above method.
As a result, from the cross-sectional image shown in FIG. 4, the area ratio SR of the coexisting region in the first electrode 101 was about 16%.

[Example 2]
By the method described above, the NOx sensor 1 in which the area ratio SR of the coexisting region was variously changed was manufactured. For each NOx sensor 1, the temperature setting of the sensor control device 169 was changed to change the temperature of the sensor element 100, and Vp1 was measured when the gas to be measured was the atmosphere.
FIG. 7 shows the relationship between the temperature of the sensor element 100 and Vp1 when the area ratio SR of the coexistence region is changed.
Here, the horizontal axis of FIG. 7 indicates the temperature change ΔT when the predetermined temperature TM of the sensor element 100 is used as a reference (0). For example, "-40 ° C" represents when the temperature of the sensor element 100 is 40 ° C lower than the predetermined temperature TM. The vertical axis of FIG. 7 shows the change ΔVp1 of Vp1 when Vp1 at a predetermined temperature TM of the sensor element 100 having an area ratio SR of 16% in the coexistence region is used as a reference (V0). For example, "5 mV" represents when Vp1 of the sensor element 100 is 5 mV higher than V0.

As shown in FIG. 7, when the area ratio SR of the coexistence region was 16%, 27%, and 33%, respectively, Vp1 decreased as the temperature of the sensor element 100 decreased. This is because the diffusion rate of the gas to be measured (atmosphere) decreases as the temperature decreases, which shows the tendency of a normal sensor element.
On the other hand, when the area ratio SR of the coexistence region is 12% and 5%, respectively, Vp1 hardly decreases even if the temperature of the sensor element 100 decreases (SR = 12%), or Vp1 increases on the contrary (SR =). 5%). As a result, when SR = 12% and 5%, the internal resistance of the first electrode 101 increases and the voltage (Vp1) of the first pump cell 83 increases when the temperature drops due to the influence of disturbance or the like. It is considered that the specific gas, which is the component to be measured in the measurement gas, is decomposed and the measurement accuracy is lowered.
Further, when SR = 16%, 27%, and 33%, the factor contributing to the temperature dependence of Vp1 is largely influenced by the diffusion rate of the gas to be measured, while the factor due to the SR value is small. By the correction, the measurement accuracy can be easily improved in consideration of the temperature dependence of Vp1. However, when SR = 12% and 5%, which are different from this tendency, the influence of factors such as internal resistance that greatly change depending on the SR value becomes large on the temperature dependence of Vp1, so the measurement accuracy depends on the design and correction. It becomes difficult to increase.

[Example 3]
By the method described above, the NOx sensor 1 in which the area ratio SR of the coexisting region was variously changed was manufactured. For each NOx sensor 1, the temperature of the sensor element 100 was controlled to a predetermined temperature, and Ip1 during a predetermined time was measured using the gas to be measured as the atmosphere.
FIG. 8 shows Ip1 (to be exact, the following noise level NL) when the area ratio SR of the coexistence region is changed.
Here, the vertical axis NL (Noise Level) in FIG. 8 is the difference between the maximum value and the minimum value of Ip1 during a predetermined time. The thick line along the horizontal axis indicates the permissible threshold value TH of NL (when NL becomes higher than this, the measurement accuracy of oxygen decreases). For example, "0.01 mA" represents when the NL of the sensor element 100 is 0.01 mA higher than the SH.

As shown in FIG. 8, when the area ratio SR of the coexisting region was 33%, the NL rapidly increased and exceeded the permissible threshold TH. As described above, when the fluctuation (noise) of Ip1 is large, oscillation due to a disturbance factor (such as a sudden change in oxygen concentration in the gas to be measured or a gas flow velocity) is likely to occur. That is, it is considered that the difference in responsiveness between the first electrode 101 including the coexistence region 207 and the third electrode 109 becomes large, and oscillation becomes easy.
From the above, it was found that when the area ratio SR is 15.5% or more and less than 30%, the decrease in measurement accuracy can be suppressed and the oscillation can also be suppressed.

1 NOx sensor (gas sensor)
5 Main metal fittings 69 1st solid electrolyte body (solid electrolyte body)
83 First pump cell (pump cell)
85 Reference cell (reference cell)
87 Second pump cell (NOx detection cell)
89 1st measurement room (measurement room)
99 Second electrode (outer electrode)
100 NOx sensor element (sensor element)
101 First electrode (inner electrode)
169 Sensor control device (gas sensor control unit)
201 Cross section 203 Solid electrolyte region 205 Noble metal region 207 Coexistence region

Claims (5)

The measurement room and
A solid electrolyte, an inner electrode formed on the surface of the solid electrolyte and exposed to the measurement chamber, and an outer electrode formed on the surface of the solid electrolyte and arranged outside the measurement chamber. A pump cell that adjusts the oxygen concentration in the measurement chamber by pumping and pumping oxygen in the gas to be measured that is introduced into the measurement chamber.
A sensor element having a reference cell that generates a voltage corresponding to the oxygen concentration in the gas to be measured in the measurement chamber.
At least one of the inner electrode and the outer electrode contains a noble metal and a component of the solid electrolyte, and when the cross section is observed along the thickness direction, the noble metal region composed of the noble metal is formed. It has a solid electrolyte region composed of the components of the solid electrolyte body and a coexistence region in which the noble metal and the components of the solid electrolyte body coexist.
In the cross section of the at least one electrode, the area ratio SR of the coexisting region represented by {the coexisting region / (the noble metal region + the solid electrolyte region + the coexisting region)} is 15.5% or more, 30 A sensor element, characterized in that it is less than%. It further has a NOx detection cell for measuring the concentration of nitrogen oxides in the gas to be measured after adjusting the oxygen concentration.
The at least one electrode comprises at least the inner electrode.
The sensor element according to claim 1, wherein the area ratio SR of the coexisting region in the cross section of the inner electrode is 15.5% or more and less than 30%. The sensor element according to claim 1 or 2, wherein the area ratio SR of the coexistence region is 16% or more and 27% or less. A gas sensor comprising the sensor element according to any one of claims 1 to 3 and a main metal fitting for holding the sensor element. The gas sensor according to claim 4 and
A gas sensor control unit connected to the gas sensor is provided.
The gas sensor control unit is a gas sensor unit characterized in that the current flowing through the pump cell is feedback-controlled so that the potential of the reference cell becomes constant.

Images