Sensor element, gas sensor, and manufacturing method of sensor element

Abstract

To suppress excessive low frequency shift of frequency characteristics caused by rich aging.SOLUTION: A sensor element 7 comprises a measurement chamber into which a measured gas is introduced, and a cell pumping oxygen from the measurement chamber to the outside of the element and pumping oxygen into the measurement chamber from the outside of the element. The cell includes a solid electrolyte, a first electrode 101 formed on the solid electrolyte and facing the measurement chamber, and a second electrode 99 formed on the solid electrolyte and facing the element outside or a space communicating with the element outside. The first electrode 101 includes a noble metal layer made of noble metal, a ceramic layer made of ceramics contained in the solid electrolyte, and a mixed layer 207 including the noble metal and the ceramics which are mixed therein. The noble metal is unevenly distributed in the mixed layer 207.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a sensor element, a gas sensor, and a method for manufacturing a sensor element. [Background technology]

[0002] A conventional sensor known from Japanese Patent Application Laid-Open No. 2019-158866 (Patent Document 1) includes a measurement chamber into which a gas to be measured is introduced, a pump cell, a reference oxygen chamber, and a reference cell. The pump cell pumps oxygen from the measurement chamber to the outside and pumps oxygen from the outside into the measurement chamber. The reference oxygen chamber contains an atmosphere with a predetermined oxygen concentration. The reference cell generates a voltage Vs according to the difference in oxygen concentration between the measurement chamber and the reference oxygen chamber. The pump cell includes a first solid electrolyte body, a first electrode formed on the first solid electrolyte body and facing the measurement chamber, and a second electrode formed on the first solid electrolyte body and facing the outside.

[0003] The first electrode contains a precious metal and a ceramic contained in a first solid electrolyte body. The first electrode is formed by performing a rich aging treatment on a first electrode precursor. The rich aging treatment causes the first electrode precursor to undergo alteration, forming a coexistence region in which the precious metal and ceramic coexist. The formation of the coexistence region increases the number of three-phase interfaces between the precious metal, solid electrolyte, and gas, improving the reactivity between the first electrode and the gas and improving the activity of the first electrode. The rich aging treatment is performed, for example, by applying only a positive voltage between the first electrode precursor and the second electrode in a rich atmosphere. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-158866 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above sensor, the rich aging process causes the frequency characteristics to shift excessively to low frequencies. The inventors of the present application speculate that the reason for this is as follows: During the rich aging process, the applied positive voltage rapidly removes oxygen from the first electrode, destroying the crystal structure and leaving Zr and Pt in a uniformly dispersed fine powder state. In this state, the internal resistance is reduced due to the large number of interfaces, but the crystal structure is very dense, limiting the paths for oxygen to pass through, which is thought to be why the frequency characteristics shift excessively to low frequencies. [Means for solving the problem]

[0006] The sensor element of the present disclosure is a sensor element comprising: a measuring chamber into which a gas to be measured is introduced; and a cell that pumps oxygen from the measuring chamber to the outside of the element and pumps oxygen from the outside of the element into the measuring chamber, wherein the cell comprises: a solid electrolyte body; a first electrode formed on the solid electrolyte body and facing the measuring chamber; and a second electrode formed on the solid electrolyte body and facing the outside of the element or a space communicating with the outside of the element, wherein the first electrode has a precious metal layer made of a precious metal, a ceramic layer made of a ceramic contained in the solid electrolyte body, and a mixed layer in which the precious metal and the ceramic are mixed, and the precious metal is unevenly distributed in the mixed layer. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to suppress an excessive low-frequency shift in frequency characteristics due to rich aging treatment. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view showing the internal structure of a NOx sensor. [Figure 2] FIG. 2 is a perspective view showing the sensor element with a portion of the sensor element in the axial direction omitted. [Figure 3] FIG. 3 is an explanatory diagram showing an enlarged view of the internal structure of the sensor element cut in the thickness direction at the tip end side. [Figure 4]FIG. 4 is an explanatory diagram showing the electrical configuration of the NOx sensor. [Figure 5] FIG. 5 is a graph showing the application of a negative voltage immediately after the application of a positive voltage in the rich aging treatment. [Figure 6] FIG. 6 is a graph showing the application of only a positive voltage during rich aging treatment. [Figure 7] FIG. 7 is an explanatory diagram showing how oxygen ions move when a voltage is applied. [Figure 8] FIG. 8 is a TEM photograph of the mixed layer. [Figure 9] FIG. 9 is a graph showing frequency characteristics. [Figure 10] FIG. 10 is a graph showing the Lorenz curve. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. [1] The sensor element of the present disclosure includes a measurement chamber into which a gas to be measured is introduced, and a cell that pumps oxygen from the measurement chamber to the outside of the element and pumps oxygen from the outside of the element to the measurement chamber, the cell including a solid electrolyte body, a first electrode formed on the solid electrolyte body and facing the measurement chamber, and a second electrode formed on the solid electrolyte body and facing the outside of the element or a space communicating with the outside of the element, the first electrode including a precious metal layer made of a precious metal, a ceramic layer made of a ceramic contained in the solid electrolyte body, and a mixed layer containing the precious metal and the ceramic, the precious metal being unevenly distributed in the mixed layer. Whether the precious metal is unevenly distributed is determined according to the following measurement procedure.

[0010] <Measurement procedure> (1) The mixed layer is observed by STEM with a field of view of 425 nm × 320 nm. (2) Draw 10 line segments that cross the field of view horizontally, including at least the mixed layer at both ends and in the center. (3) Based on the EDS mapping of platinum in the same field of view, create a Lorentz curve for platinum in the line segment (see "Procedure for creating a Lorentz curve" below). (4) Calculate the Gini coefficient from the Lorenz curve (follow the "Method for calculating the Gini coefficient" below). (5) Calculate the Gini coefficient for all 10 lines, and if any line exceeds 0.22, it is considered to be “unevenly distributed.”

[0011] <Procedure for creating a Lorenz curve> (1) Convert the platinum EDS mapping image into numerical information (quantify the brightness of the platinum dots). (2) The numerical information in (1) is extracted by narrowing it down to the measurement area (on the line segment) and sorted in ascending order. By sorting in ascending order, the numerical values ​​can be sorted from smallest to largest. (3) The cumulative values ​​of the ascending data in (2) are calculated one by one to obtain the "cumulative sensitivity" and "cumulative number of dots." (4) Divide the "cumulative sensitivity" and "cumulative dot count" by their respective totals to obtain the "relative cumulative sensitivity" and "relative cumulative dot count," and plot these on a graph. If the platinum were distributed completely evenly, the values ​​would be uniform, so the plot would be a straight line (a perfectly even line), but in reality, the platinum is unevenly distributed, so there are variations in the platinum density, resulting in a curve. <Method of calculating the Gini coefficient> It is calculated using the following formula. ("Integral of the Perfect Equal Line" - "Integral of the Lorenz Curve" / "Integral of the Perfect Equal Line"

[0012] During rich aging, the first electrode precursor undergoes a transformation, forming a mixed layer containing precious metals and ceramics. When a positive voltage is applied between the first electrode precursor and the second electrode, oxygen ions migrate from the first electrode precursor to the second electrode, destroying the crystal structure of the first electrode and uniformly dispersing the precious metals and ceramics into fine particles. When a negative voltage is applied immediately afterward, oxygen ions migrate from the second electrode to the first electrode precursor, recrystallizing and segregating the finely pulverized precious metals and ceramics, resulting in a coarse crystal structure. This facilitates oxygen migration within the mixed layer, adjusting the frequency characteristics to the desired level. This suppresses excessive low-frequency shifts in frequency characteristics due to rich aging, thereby suppressing low-frequency oscillation and noise during sensor response.

[0013] [2] The gas sensor of the present disclosure may be a gas sensor including the sensor element described in [1] above and a housing surrounding the sensor element.

[0014] [3] A method for manufacturing a sensor element according to the present disclosure includes a measuring chamber into which a gas to be measured is introduced, and a cell that pumps oxygen from the measuring chamber to the outside of the element and pumps oxygen from the outside of the element to the measuring chamber. The cell includes a solid electrolyte body, a first electrode formed on the solid electrolyte body and facing the measuring chamber, and a second electrode formed on the solid electrolyte body and facing the outside of the element or a space communicating with the outside of the element. The first electrode has a precious metal layer made of a precious metal, a ceramic layer made of a ceramic contained in the solid electrolyte body, and a mixed layer in which the precious metal and the ceramic are mixed. The first electrode is formed by performing a rich aging treatment on a first electrode precursor. The rich aging treatment is performed by applying a positive voltage between the first electrode precursor and the second electrode in a rich atmosphere and then immediately applying a negative voltage.

[0015] [Details of the embodiments of the present disclosure] Specific examples of connectors according to the present disclosure will be described below with reference to the drawings. For ease of explanation, some components may be exaggerated or simplified in the drawings. The dimensional ratios of the components may differ between the drawings. Furthermore, the terms "parallel," "perpendicular," and "orthogonal" used in this specification do not necessarily mean strictly parallel, perpendicular, or orthogonal, but also include roughly parallel, perpendicular, or orthogonal within the scope of the functions and effects of the present embodiment.

[0016] <Embodiment> An embodiment will be described with reference to FIGS. 1 to 9. A NOx sensor (an example of a gas sensor) 1 of this embodiment is used in a urea SCR (Selective Catalytic Reduction) system that purifies nitrogen oxides (NOx) contained in exhaust gas (an example of a measured gas) emitted from an automobile diesel engine (an example of an internal combustion engine). The urea SCR system purifies nitrogen oxides contained in exhaust gas by causing ammonia and nitrogen oxides to chemically react with each other to reduce the nitrogen oxides to nitrogen. In a urea SCR system, if the amount of ammonia supplied relative to the nitrogen oxides becomes excessive, there is a risk that unreacted ammonia will remain in the exhaust gas and be released to the outside. The NOx sensor 1 measures the concentrations of nitrogen oxides and ammonia contained in the exhaust gas.

[0017] (1) Overall configuration of NOx sensor 1 The overall configuration of the NOx sensor 1 will be described with reference to Figures 1 and 2. In the following, the lower side in Figure 1 will be referred to as the front end side of the NOx sensor 1, and the upper side in Figure 1 will be referred to as the rear end side of the NOx sensor 1.

[0018] 1, the NOx sensor 1 includes a metal shell (an example of a housing) 5, a sensor element 7, a ceramic sleeve 9, an insulating separator 13, and six lead frames 15. Note that only some of the six lead frames 15 are shown in FIG.

[0019] Each component will be described below. As shown in Figs. 1 and 2, the sensor element 7 is a plate-shaped laminated member extending in the axial direction. The sensor element 7 penetrates the metal shell 5. The tip side of the sensor element 7 faces the exhaust gas to be measured. A detection section 17 is formed on the tip side of the sensor element 7. The detection section 17 is covered with a protective layer 18, as shown in Fig. 1.

[0020] 2, electrode pads 23, 25, 27, 29, 31, and 33 are formed on the rear end side of the sensor element 7. The electrode pads 23, 25, and 27 are formed on a first plate surface 19, which is one of the outer surfaces of the sensor element 7. The electrode pads 29, 31, and 33 are formed on a second plate surface 21, which is the surface of the outer surface of the sensor element 7 opposite to the first plate surface 19.

[0021] The ceramic sleeve 9 has a cylindrical shape and is disposed so as to surround the sensor element 7 in the radial direction.

[0022] The insulating separator 13 is made of an insulating material such as alumina. The insulating separator 13 has an element insertion hole 11 that penetrates in the axial direction. The element insertion hole 11 surrounds at least a portion of the sensor element 7 and the lead frame 15.

[0023] 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 each of the electrode pads 23 to 33 of the sensor element 7. The lead frame 15 is also electrically connected to a lead wire 35 that is arranged inside the NOx sensor 1 from the outside, and forms a current path for a current to flow between the electrode pads 23 to 33 and an external device to which the lead wire 35 is connected.

[0024] The metal shell 5 is configured to hold the sensor element 7 inserted into the through hole 37. The sensor element 7 is held in a state in which the detection portion 17 is arranged outside the tip end side of the through hole 37 and the electrode pads 23 to 33 are arranged outside the rear end side of the through hole 37.

[0025] Inside the through hole 37, a ring-shaped ceramic holder 41, a powder-filled layer (talc ring) 43, and the above-mentioned ceramic sleeve 9 are layered in this order from the tip end to the rear end, surrounding the radial periphery of the sensor element 7.

[0026] A crimping ring 49 is disposed between the ceramic sleeve 9 and the rear end portion 47 of the metallic shell 5. A metal cup 51 is disposed between the ceramic holder 41 and the shelf portion 39 of the metallic shell 5. The rear end portion 47 is crimped so as to press the ceramic sleeve 9 against the front end side via the crimping ring 49.

[0027] A cylindrical protector 52 made of, for example, stainless steel is disposed on the tip side of the metallic shell 5 so as to cover the tip side of the sensor element 7. The protector 52 has ventilation holes 53 that allow exhaust gas to pass through. The protector 52 has a double protector structure consisting of an inner protector and an outer protector.

[0028] An outer cylinder (an example of a housing) 55 made of, for example, stainless steel is fixed to the rear end side of the metallic shell 5. An opening 57 on the rear end side of the outer cylinder 55 is closed by a grommet 59 made of, for example, fluororubber.

[0029] The insulating separator 13 is held in the outer tube 55 with its rear end abutting against the grommet 59. The insulating separator 13 is held by a holding member 61. The holding member 61 is fixed to the inside of the outer tube 55 by crimping.

[0030] 2. Structure of sensor element 7 The structure of the sensor element 7 will be described with reference to Figs. 2 to 4. As shown in Fig. 2, the sensor element 7 has a plate-like shape with a rectangular axial cross section. The sensor element 7 has a structure in which an element portion 63 and a heater 65 are stacked. The element portion 63 and the heater 65 are each formed in a plate shape extending in the axial direction.

[0031] The configuration of the tip side of the sensor element 7 is shown in Fig. 3. The sensor element 7 has a structure in which, from top to bottom in Fig. 3, an insulating layer 67, a first solid electrolyte body 69, an insulating layer 71, a second solid electrolyte body 73, an insulating layer 75, a third solid electrolyte body 77, and insulating layers 79 and 81 are stacked. Of these, the insulating layer 67, the first solid electrolyte body 69, the insulating layer 71, the second solid electrolyte body 73, the insulating layer 75, and the third solid electrolyte body 77 constitute the element section 63. The sensor element 7 includes a first pump cell 83, a reference cell 85, and a second pump cell 87.

[0032] A first measuring 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 measuring chamber 89 is the inlet. A first diffusion resistance member 91 is disposed at this inlet. Exhaust gas GM is introduced into the first measuring chamber 89 from the outside through the first diffusion resistance member 91. A second diffusion resistance member 93 is disposed at the end of the first measuring chamber 89 opposite the inlet.

[0033] 3, 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. A portion of the second solid electrolyte body 73 corresponding to the second measurement chamber 95 is cut out.

[0034] The first to third solid electrolyte bodies 69, 73, and 77 are each primarily composed of zirconia, which has oxygen ion conductivity. The insulating layers 67, 71, 75, 79, and 81 are each primarily composed of alumina. The first and second diffusion resistance members 91 and 93 are each made of a porous material such as alumina. The term "primary component" refers to a component whose content in the ceramic layer is 50% by mass or more.

[0035] A resistance heating element 97 is embedded between the insulating layers 79 and 81. The resistance heating element 97 extends in the left-right direction in FIG. 3. The resistance heating element 97 is made of, for example, platinum. The insulating layers 79 and 81 and the resistance heating element 97 constitute the heater 65. The heater 65 heats the sensor element 7 to a predetermined activation temperature and increases the oxygen ion conductivity of the first to third solid electrolyte bodies 69, 73, and 77, thereby stabilizing operation.

[0036] 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 therebetween.

[0037] The first electrode 101 is formed on the first solid electrolyte body 69. The first electrode 101 faces the first measuring chamber 89. The first electrode 101 is porous. The first electrode 101 includes platinum, zirconia, and a mixed layer containing platinum and zirconium. The volume ratio of the mixed layer in the first electrode 101 is 15% by volume or more. The zirconia corresponds to the ceramic contained in the first solid electrolyte body 69. The platinum corresponds to the precious metal.

[0038] The second electrode 99 is formed on the first solid electrolyte body 69. The second electrode 99 faces the outside of the sensor element 7. The second electrode 99 is mainly composed of platinum. 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 material that allows gases such as oxygen to pass through. An example of this porous material is alumina.

[0039] The reference cell 85 includes a second solid electrolyte body 73, a third electrode 109, and a fourth electrode 111. The third electrode 109 is formed on the second solid electrolyte body 73. The fourth electrode 111 is formed on the second solid electrolyte body 73. The third electrode 109 and the fourth electrode 111 sandwich the second solid electrolyte body 73. The third electrode 109 faces the first measuring chamber 89. The fourth electrode 111 faces a 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.

[0040] The reference oxygen chamber 113 is a cutout portion of the insulating layer 75. The reference oxygen chamber 113 is a space surrounded by the second solid electrolyte body 73, the third solid electrolyte body 77, and the insulating layer 75. The interior of the reference oxygen chamber 113 contains an atmosphere with a predetermined oxygen concentration.

[0041] 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 each formed on one surface of the third solid electrolyte body 77. 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 by a porous insulating protective layer 165. The reference oxygen chamber 113 includes a void 167, which is an unfilled space.

[0042] 3. Electrical configuration of NOx sensor 1 The electrical configuration of the NOx sensor 1 that controls the operation of the sensor element 7 will be described with reference to Fig. 4. The NOx sensor 1 has a microcomputer 171, an electrical circuit section 173, etc. The microcomputer 171 has a CPU 175 that performs various calculations, a RAM 177 that stores the calculation results, etc., and a ROM 179 that stores programs to be executed by the CPU 175, etc.

[0043] The microcomputer 171 also includes an A / D converter 181, a signal input / output unit 185, a timer clock (not shown), etc. 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.

[0044] The electric circuit unit 173 is composed of a reference voltage comparison circuit 187, an Ip1 drive circuit 189, a Vs detection circuit 191, 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. Under the control of the microcomputer 171, the electric circuit unit 173 detects the NOx concentration in the exhaust gas GM using the sensor element 7.

[0045] The first electrode 101, the third electrode 109, and the fifth electrode 115 are connected to a reference potential, and one electrode of the resistance heating element 97 is grounded.

[0046] 4.NOx concentration detection process The process of detecting the NOx concentration in the exhaust gas GM, performed by the NOx sensor 1, will now be described. The heater drive circuit 199 applies a drive current to the resistance heating element 97. The resistance heating element 97 rises in temperature, heating and activating the first to third solid electrolyte bodies 69, 73, and 77. This causes the first pump cell 83, the reference cell 85, and the second pump cell 87 to operate.

[0047] The exhaust gas GM is introduced into the first measuring chamber 89 while the flow rate is limited by the first diffusion resistance portion 91. Here, the Icp supply circuit 193 passes a weak current Icp from the fourth electrode 111 to the third electrode 109 in the reference cell 85. As a result, oxygen in the exhaust gas GM can receive electrons from the third electrode 109 in the first measuring chamber 89, which is the negative electrode, and become oxygen ions. These ions flow through the second solid electrolyte body 73 and migrate into the reference oxygen chamber 113. In other words, the current Icp passing between the third electrode 109 and the fourth electrode 111 sends oxygen from the first measuring chamber 89 into the reference oxygen chamber 113.

[0048] 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 first measuring chamber 89 and the reference oxygen chamber 113. The Vs detection circuit 191 compares the detected voltage Vs with a reference voltage (425 mV) using the reference voltage comparison circuit 187, and outputs the comparison result to the Ip1 drive circuit 189. If the oxygen concentration in the first measuring chamber 89 is adjusted so that the voltage Vs is constant around 425 mV, the oxygen concentration in the exhaust gas GM in the first measuring chamber 89 will be a predetermined value (for example, 10 -8 ~10 -9 This will bring you closer to ATM.

[0049] When the oxygen concentration of the exhaust gas GM introduced into the first measuring chamber 89 is lower than a predetermined value, the Ip1 drive circuit 189 passes a current Ip1 through the first pump cell 83 so that the second electrode 99 side becomes negative, thereby pumping oxygen into the first measuring chamber 89 from outside the sensor element 7. On the other hand, when the oxygen concentration of the exhaust gas GM introduced into the first measuring 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 negative, thereby pumping oxygen from inside the first measuring chamber 89 to outside the sensor element 7.

[0050] The exhaust gas GM, whose oxygen concentration has been adjusted in the first measuring chamber 89, is introduced into the second measuring chamber 95 via the second diffusion resistance portion 93. NOx in the exhaust gas GM that comes into contact with the fifth electrode 115 in the second measuring chamber 95 is decomposed into N2 and O2 on the fifth electrode 115 by applying a voltage Vp2 between the sixth electrode 117 and the fifth electrode 115 using the Vp2 application circuit 197. The decomposed oxygen becomes oxygen ions that flow through the third solid electrolyte body 77 and migrate into the reference oxygen chamber 113. Therefore, the current flowing through the second pump cell 87 indicates a value corresponding to the NOx concentration.

[0051] The NOx sensor 1 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 determined in advance, and a map or the like is prepared in advance, and the measured current Ip2 is referenced to this map to determine the NOx concentration.

[0052] 5. Manufacturing method of NOx sensor 1 Ceramic sheets are prepared as raw materials for the insulating layer 67, the first solid electrolyte body 69, the second solid electrolyte body 73, the third solid electrolyte body 77, and the insulating layers 79 and 81. Through holes and the like are formed in the ceramic sheets as appropriate. Additionally, the insulating layers 71 and 75 are formed on the ceramic sheets by screen printing.

[0053] Next, a paste containing raw materials for the electrodes is applied to the surface of the corresponding ceramic sheet to form the electrodes 99, 101, 109, 111, 115, and 117. The paste for forming the electrodes contains platinum and ZrO2.

[0054] Next, ceramic sheets are stacked to form a laminate, and this laminate is fired. At this time, a first electrode precursor 101A is formed in the portion that will later become the first electrode 101. The first electrode precursor 101A contains platinum and ZrO2. When the mass of platinum contained in the first electrode precursor 101A is taken as 100 parts by mass, the mass of ZrO2 contained in the first electrode precursor 101A is 22 parts by mass. Furthermore, in the first electrode precursor 101A, the volume occupied by platinum is 56% by volume, and the volume occupied by ZrO2 is 44% by volume.

[0055] Next, the first electrode precursor 101A is subjected to a rich aging treatment. The conditions for the rich aging treatment are as follows, for example. Atmosphere of first electrode precursor 101A: Rich atmosphere Positive voltage between the first electrode precursor 101A and the second electrode 99 and its treatment time: 1.6 V x 40 seconds Negative voltage between the first electrode precursor 101A and the second electrode 99 and its treatment time: −0.45 V × 30 seconds

[0056] Under the above conditions, while first electrode precursor 101A is heated by heater 65, a negative voltage is applied to first electrode precursor 101A immediately after a positive voltage is applied. This application of a negative voltage is hereinafter also referred to as after-control. FIG. 5 shows a graph illustrating the application of a negative voltage immediately after a positive voltage is applied, and FIG. 6 shows a graph illustrating the application of only a positive voltage. The thick dashed line indicates the current value during rich aging treatment, the thick solid line indicates the voltage value corresponding to the current value during rich aging treatment, the thin dashed line indicates the Ip1 current value, and the thin solid line indicates the Vp1 voltage value corresponding to the Ip1 current value. The graph in FIG. 5 also shows that a negative voltage is applied immediately after a positive voltage is applied.

[0057] A rich atmosphere is an atmosphere in which the proportion of oxygen is low compared to the theoretical air-fuel ratio (λ=1), which is the mixture ratio of air and fuel that allows ideal complete combustion. In the rich aging treatment, for example, the first electrode precursor 101A is heated using a heater. The heater used may be the heater 65 or an external heater. The rich aging treatment causes the deterioration of first electrode precursor 101A to occur, resulting in the generation of mixed layer 207, and the formation of first electrode 101.

[0058] 6. Surface observation of sensor element 7 Before and after the after-control, a cross section of the first electrode 101 along the longitudinal direction of the sensor element 7 was observed under magnification by a TEM.

[0059] 7(A) shows a first electrode precursor 101A before the rich aging treatment, a second electrode 99, and a first solid electrolyte body 69 located between the first electrode precursor 101A and the second electrode 99. FIG. 7(B) shows the application of a positive voltage during the rich aging treatment. The first electrode precursor 101A and the second electrode 99 have a platinum layer made of platinum 205 and a zirconia layer made of zirconia 203 contained in the first solid electrolyte body 69.

[0060] When a positive voltage is applied, oxygen ions 201 migrate from first electrode precursor 101A to second electrode 99, as shown in Figure 7(B), and a mixed layer 207 consisting of platinum 205, zirconia 203, platinum, and zirconium is formed on first electrode precursor 101A. At this time, the applied voltage causes oxygen ions 201 to be rapidly removed from first electrode precursor 101A, destroying the crystal structure and uniformly dispersing zirconia 203 and platinum 205 into fine powder. In this state, the internal resistance can be reduced due to the large number of interfaces, but the crystal structure is very dense, making it difficult for oxygen ions 201 to migrate, resulting in an excessively low-frequency shift in the frequency characteristics (see the dashed line in Figure 9).

[0061] Next, when a negative voltage is applied, oxygen ions 201 migrate from second electrode 99 to first electrode precursor 101A, and the platinum 205 and zirconia 203 that were pulverized in first electrode precursor 101A recrystallize and segregate, resulting in a coarse crystal structure. This facilitates the migration of oxygen ions 201 within mixed layer 207, and the frequency characteristics are adjusted to the desired value (see the solid line in FIG. 9).

[0062] 7. Surface observation before and after control The top row of Figure 8 is a TEM photograph taken before the after-treatment control, and the bottom row of Figure 8 is a TEM photograph taken after the after-treatment control. The photograph on the right is the original TEM photograph, the photograph on the left is a photograph in which zirconia 203 was extracted by EDS mapping, and the center photograph is a photograph in which platinum 205 was extracted by EDS mapping. In the photograph on the top left, zirconia was detected in the white area at the bottom left, but platinum was not, indicating that this area is zirconia 203. In the photograph in the top center, both platinum and zirconia were detected in the area excluding the black areas at the bottom left and top right, indicating that this is a mixed layer 207 in which platinum and zirconia are mixed. As such, both platinum and zirconia were detected uniformly before the after-treatment control, indicating that both zirconia and platinum were uniformly present in the mixed layer. On the other hand, in the photographs on the bottom left and bottom center, white and black areas are finely separated and mixed. Therefore, it is clear that after the after-treatment, the zirconia 203 and platinum 205 are unevenly distributed.

[0063] Next, a measurement method for specifically determining whether zirconia and platinum are unevenly distributed in the mixed layer will be described. Whether or not they are "unevenly distributed" is determined according to the following measurement procedure. <Measurement procedure> (1) The mixed layer is observed by STEM with a field of view of 425 nm × 320 nm. (2) Draw 10 line segments that cross the field of view horizontally, including at least the mixed layer at both ends and in the center. (3) Based on the EDS mapping of platinum in the same field of view, create a Lorentz curve for platinum in the line segment (see "Procedure for creating a Lorentz curve" below). (4) Calculate the Gini coefficient from the Lorenz curve (follow the "Method for calculating the Gini coefficient" below). (5) Calculate the Gini coefficient for all 10 lines, and if any line exceeds 0.22, it is considered to be “unevenly distributed.”

[0064] <Procedure for creating a Lorenz curve> The procedure for creating the Lorenz curve shown in Figure 10 is shown in (1) to (4) below. (1) Convert the platinum EDS mapping image into numerical information (quantify the brightness of the platinum dots). (2) The numerical information in (1) is extracted by narrowing it down to the measurement area (on the line segment) and sorted in ascending order. By sorting in ascending order, the numerical values ​​can be sorted from smallest to largest. (3) The cumulative values ​​of the ascending data in (2) are taken point by point to obtain the "cumulative sensitivity" and "cumulative number of dots." (4) Divide the "cumulative sensitivity" and "cumulative dot count" by their respective totals to obtain the "relative cumulative sensitivity" and "relative cumulative dot count," and plot these on a graph. If the platinum were distributed completely evenly, the values ​​would be uniform, so the plot would be a straight line (a perfectly even line), but in reality, the platinum is unevenly distributed, so there are variations in the platinum density, resulting in a curve.

[0065] <Method of calculating the Gini coefficient> It is calculated using the following formula. ("Integral of the Perfect Equal Line" - "Integral of the Lorenz Curve" / "Integral of the Perfect Equal Line" The Gini coefficient quantifies the uneven distribution of platinum. A larger Gini coefficient indicates greater uneven distribution. The horizontal axis of Figure 10 represents the relative cumulative dot count, and the vertical axis of Figure 10 represents the relative cumulative sensitivity. Line A represents the line of perfect uniformity. Curve B represents the graph without after-treatment, and curve C represents the graph with after-treatment. The arc-shaped region enclosed by line A and curve B or curve C is region F, and the right-angled triangular region enclosed by line A, line D, and line E is region G. The area of ​​region F can be calculated by subtracting the integral of curve B or curve C from the integral of line A, and the area of ​​region G is the integral of line A. The Gini coefficient can be calculated by dividing the area of ​​region F by the area of ​​region G.

[0066] [Table 1]

[0067] [Table 2]

[0068] The comparative examples in Table 1 had a maximum of 0.207, while the examples in Table 2 had a minimum of 0.209. Since the average of the comparative examples +3s = 0.221, 0.22 can be set as the threshold, and if there is a line segment with a Gini coefficient exceeding 0.220, it can be deemed "unevenly distributed." More preferably, 0.24 can be set as the threshold, and if there is a line segment with a Gini coefficient exceeding 0.240, it can be deemed "unevenly distributed." In order to exclude segments that contain only platinum or only zirconia, an upper limit of 0.425 (average of the examples + 5s) may be used. It is desirable to have segments with a Gini coefficient between 0.220 (0.240) and 0.425. As can be seen by comparing the Gini coefficients in Tables 1 and 2, the Gini coefficient of 0.208 separates the cases without after-treatment from those with after-treatment. It can be seen that the Gini coefficient with after-treatment is larger than the Gini coefficient without after-treatment.

[0069] 8. Frequency characteristics of sensor element 7 9 is a graph showing the frequency characteristics immediately after application of a positive voltage (dashed line) and the frequency characteristics after after-control (solid line). In the NOx sensor 1 of this embodiment, the target frequency band where the main reaction switches between the circuit and the element is around 4 to 7 Hz. Looking at the graph after after-control, it can be seen that the phase changes significantly around 4 to 7 Hz, and therefore the target frequency characteristics are obtained.

[0070] 8. Effects of the embodiment The gas sensor element 7 of the present disclosure is a sensor element 7 including a first measuring chamber 89 into which exhaust gas GM is introduced, and a first pump cell 83 that pumps oxygen from the first measuring chamber 89 to the outside of the element and pumps oxygen from the outside of the element to the first measuring chamber 89. The first pump cell 83 includes a first solid electrolyte body 69, a first electrode 101 formed on the first solid electrolyte body 69 and facing the first measuring chamber 89, and a second electrode 99 formed on the first solid electrolyte body 69 and facing the outside of the element. The electrode 101 has a precious metal layer made of platinum 205, a ceramic layer made of zirconia 203 contained in the first solid electrolyte body 69, and a mixed layer 207 in which platinum 205 and zirconia 203 are mixed, and the first electrode 101 is formed by performing a rich aging treatment on a first electrode precursor 101A, and the rich aging treatment is performed by applying a positive voltage between the first electrode precursor 101A and the second electrode 99 in a rich atmosphere and then applying a negative voltage immediately thereafter.

[0071] The rich aging process causes the first electrode precursor 101A to degrade, forming a mixed layer 207 containing platinum 205 and zirconia 203. When a positive voltage is applied between the first electrode precursor 101A and the second electrode 99, oxygen ions 201 migrate from the first electrode 101A to the second electrode 99, destroying the crystal structure of the first electrode precursor 101A and uniformly dispersing the platinum 205 and zirconia 203 into fine powder. When a negative voltage is applied immediately thereafter, oxygen ions 201 migrate from the second electrode 99 to the first electrode precursor 101A, recrystallizing and segregating the pulverized platinum 205 and zirconia 203, resulting in a coarse crystal structure. This facilitates oxygen migration within the mixed layer 207, adjusting the frequency characteristics to the desired level. This suppresses excessive low-frequency shifts in the frequency characteristics due to the rich aging process, thereby suppressing low-frequency oscillation and noise during sensor response.

[0072] The NOx sensor 1 of the present disclosure includes the above-described sensor element 7, and a metal shell 5 and an outer cylinder 55 that surround the sensor element 7.

[0073] The manufacturing method of the sensor element 7 of the present disclosure is a manufacturing method of the sensor element 7 including a first measuring chamber 89 into which the exhaust gas GM is introduced, and a first pump cell 83 that pumps oxygen from the first measuring chamber 89 to the outside of the element and pumps oxygen from the outside of the element to the first measuring chamber 89, and the first pump cell 83 includes a first solid electrolyte body 69, a first electrode 101 formed on the first solid electrolyte body 69 and facing the first measuring chamber 89, and a second electrode 99 formed on the first solid electrolyte body 69 and facing the outside of the element. The first electrode 101 has a precious metal layer made of platinum 205, a ceramic layer made of zirconia 203 contained in the first solid electrolyte body 69, and a mixed layer 207 in which the platinum 205 and the zirconia 203 are mixed, and the first electrode 101 is formed by performing a rich aging treatment on a first electrode precursor 101A, and the rich aging treatment is performed by applying a positive voltage between the first electrode precursor 101A and the second electrode 99 in a rich atmosphere and then applying a negative voltage immediately thereafter.

[0074] <Other embodiments> Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments and can be implemented in various modified forms.

[0075] (1) The sensor of the present disclosure may be a sensor other than a NOx sensor. For example, it may be a sensor obtained by removing the second pump cell 87 from the above-described NOx sensor 1. This sensor may be a sensor that measures the oxygen concentration in the measurement gas based on the amount of current Ip1.

[0076] (2) The ceramic contained in the first solid electrolyte body 69 and the first electrode 101 may be a material other than zirconia. Examples of ceramics other than zirconia include CeO2 (ceria), ThO2 (thoria), HfO2, and Bi2O3. The precious metal contained in the first electrode 101 may be a material other than platinum.

[0077] (3) A member that is not formed on the first solid electrolyte body 69 may be present on the outer periphery of the first electrode 101 .

[0078] (4) In the above embodiment, the second electrode 99 faces the outside of the element, but the second electrode may face a space that communicates with the outside of the element.

[0079] (5) The function of one component in the above embodiments may be shared among multiple components, or the functions of multiple components may be performed by one component. Also, part of the configuration of the above embodiments may be omitted. Furthermore, at least part of the configuration of the above embodiments may be added to, substituted for, or the like, the configuration of another embodiment. All aspects included in the technical idea identified by the wording of the claims are embodiments of the present disclosure. [Explanation of symbols]

[0080] 1: NOx sensor (gas sensor) 5: metal shell (housing) 7: sensor element 9: ceramic sleeve 11: element insertion hole 13: insulating separator 15: lead frame 17: detection portion 19: first plate surface 21: second plate surface 23, 25, 27, 29, 31, 33: electrode pad 35: lead wire 37: through hole 39: shelf portion 41: ceramic holder 43: powder packed layer 47: rear end portion 49: crimping ring 51: metal cup 52: protector 53: vent hole 55: outer cylinder (housing) 57: opening 59: grommet 61: holding member 63: element portion 65: heater 67: insulating layer 69: first solid electrolyte body 71: insulating layer 73: second solid electrolyte body 75: insulating layer 77: third solid electrolyte body 79, 81: Insulating layer 83: First pump cell (cell) 85: Reference cell 87: Second pump cell 89: First measurement chamber (measurement chamber) 91: First diffusion resistance portion 93: Second diffusion resistance portion 95: Second measurement chamber 97: Resistance heating element 99: Second electrode 101: First electrode 101A: First electrode precursor 103: Opening 105: Porous layer 109: Third electrode 111: Fourth electrode 113: Oxygen reference chamber 115: Fifth electrode 117: Sixth electrode 165: Insulating protective layer 167: Gap 171: Microcomputer 173: Electrical circuit section 175: CPU 177: RAM 179: ROM 181: A / D converter 183: ECU 185: Signal input / output section 187: Reference voltage comparison circuit 189: Ip1 drive circuit 191: Vs detection circuit 193: Icp supply circuit 194: Resistance detection circuit 195: Ip2 detection circuit 197: Vp2 application circuit 199: Heater drive circuit 201: Oxygen ions 203: Zirconia (ceramic) 205: Platinum (precious metal) 207: Mixed layer GM: Exhaust gas (measured gas)

Claims

1. a measurement chamber into which a gas to be measured is introduced; A sensor element including a cell for pumping oxygen from the measurement chamber to the outside of the element and for pumping oxygen from the outside of the element to the measurement chamber, the cell comprises a solid electrolyte body, a first electrode formed on the solid electrolyte body and facing the measurement chamber, and a second electrode formed on the solid electrolyte body and facing the outside of the element or a space communicating with the outside of the element, the first electrode has a precious metal layer made of a precious metal, a ceramic layer made of ceramic contained in the solid electrolyte body, and a mixed layer in which the precious metal and the ceramic are mixed, The sensor element, wherein the noble metal is unevenly distributed in the mixed layer.

2. The sensor element according to claim 1; a housing surrounding the sensor element.

3. a measurement chamber into which a gas to be measured is introduced; a cell that pumps oxygen from the measurement chamber to the outside of the element and pumps oxygen from the outside of the element to the measurement chamber, the cell comprises a solid electrolyte body, a first electrode formed on the solid electrolyte body and facing the measurement chamber, and a second electrode formed on the solid electrolyte body and facing the outside of the element or a space communicating with the outside of the element, the first electrode has a precious metal layer made of a precious metal, a ceramic layer made of ceramic contained in the solid electrolyte body, and a mixed layer in which the precious metal and the ceramic are mixed, a first electrode precursor formed by a rich aging treatment, the rich aging treatment being performed by applying a positive voltage between the first electrode precursor and the second electrode in a rich atmosphere, followed immediately by applying a negative voltage;

Images