Sensor element, gas sensor, method for evaluating sensor element, and program

Abstract

A sensor element (101) is provided with: an element main body (102); a main pump unit (21) having an inner pump electrode (22) disposed in the first inner cavity (20); a measurement electrode 44; and first and second diffusion velocity control units 11, 13. The invention relates to a sensor element 101. A first diffusion rate control unit (11) and a second diffusion rate control unit (13) have path lengths L1 and L2 [cm], widths H1 and H2 [cm], a limiting current Ip [A] of a main pump unit (21), a Faraday constant F [Asec / mol], a diffusion coefficient D [cm2 / sec] of oxygen, a gas constant R [cm3. Atm / mol.K], a temperature T [K] of an inner pump electrode (22), an oxygen partial pressure Poe [atm] in a gas to be measured, and an oxygen partial pressure Pod [atm] of a first internal cavity (20). The average height t [mm] of the second diffusion velocity control units (11, 13) is 0.0035 or more.

Description

Technical Field

[0001] The present invention relates to a sensor element, a gas sensor, and a sensor element evaluation method and program. Background Art

[0002] Gas sensors that detect the concentration of specific gases, such as NOx, in target gases, such as automobile exhaust, are known. For example, Patent Document 1 describes a gas sensor comprising: a stacked body having multiple oxygen ion conductive solid electrolyte layers and a gas flow section within the stacked body through which the target gas is introduced and circulated from a gas inlet; a main pump unit comprising an inner pump electrode disposed within a first internal cavity within the gas flow section and an outer pump electrode disposed on the outer surface of the stacked body; a measuring electrode disposed downstream of the first internal cavity within the gas flow section; and a slit-shaped diffusion rate control section disposed within the gas flow section to apply diffusion resistance to the target gas entering from the outside and introduce it into the first internal cavity. To detect NOx concentration using this gas sensor, a pump current Ip0 is first passed between the inner and outer pump electrodes to adjust the oxygen concentration in the first internal cavity. Subsequently, NOx in the target gas, whose oxygen concentration has been adjusted, is reduced within the second internal cavity. Then, the concentration of NOx in the gas to be measured is detected based on the pump current Ip2 flowing when the oxygen in the second internal cavity is sucked out.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-209128 Summary of the Invention

[0006] In the aforementioned gas sensor, the cross-sectional shape of the diffusion rate control section sometimes results in excessive diffusion resistance from the exterior to the first internal cavity, causing the main pump unit (adjustment pump unit) pump current Ip0 to have an excessively high dependence on the pressure of the measured gas (static pressure dependence). Therefore, a gas sensor is desired that can suppress the excessive static pressure dependence of the pump current Ip0.

[0007] A main object of the sensor element, gas sensor, sensor element evaluation method, and program of the present invention is to provide a sensor element in which the static pressure dependency of the pump current of the adjustment pump unit is suppressed.

[0008] The sensor element, gas sensor, sensor element evaluation method, and program of the present invention employ the following means to achieve the above-mentioned main objects.

[0009] [1] The sensor element of the present invention is:

[0010] A sensor element for detecting the concentration of a specific gas in a gas to be measured, comprising:

[0011] an element body having an oxygen ion conductive solid electrolyte layer and provided with a gas flow portion therein for introducing and flowing the gas to be measured;

[0012] an adjustment pump unit having an inner electrode disposed in the oxygen concentration adjustment chamber in the measured gas flow portion and adjusting the oxygen concentration in the oxygen concentration adjustment chamber;

[0013] a measuring electrode disposed in a measuring chamber downstream of the oxygen concentration adjustment chamber in the measured gas flow portion; and

[0014] a diffusion rate control unit provided in the measured gas flow unit, which applies diffusion resistance to the measured gas from the outside and introduces the measured gas into the oxygen concentration adjustment chamber;

[0015] The path length L [cm] of the diffusion rate control section, the width H [cm] of the diffusion rate control section, the limiting current Ip [A] of the adjustment pump cell, the Faraday constant F [A·sec / mol], the diffusion coefficient D [cm] of oxygen, and the diffusion coefficient D [cm] of oxygen were used. 2 / sec], gas constant R[cm 3 ·atm / mol·K], the temperature T[K] of the inner electrode, the oxygen partial pressure Poe[atm] in the measured gas, and the oxygen partial pressure Pod[atm] in the oxygen concentration adjustment chamber, and the height t[mm] of the diffusion rate control portion obtained by using formula (A) is greater than 0.0035.

[0016] t=L / H×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10(A)

[0017] In the sensor element of the present invention, the path length L [cm] of the diffusion rate control portion, the width H [cm] of the diffusion rate control portion, the limiting current Ip [A] of the adjustment pump unit, the Faraday constant F [A·sec / mol], the diffusion coefficient D [cm] of oxygen are used. 2 / sec], gas constant R[cm 3·atm / mol·K], the temperature of the inner electrode T[K], the oxygen partial pressure Poe[atm] in the gas to be measured, and the oxygen partial pressure Pod[atm] in the oxygen concentration adjustment chamber. The height t[mm] of the diffusion rate control unit obtained by formula (A) is 0.0035 or more. This can suppress the cross-sectional area [cm], which is the product of the width H[cm] and the height t / 10[cm] of the diffusion rate control unit. 2 ] is too small, in other words, it is possible to suppress the diffusion resistance from the outside to the oxygen concentration adjustment chamber from being too large. Therefore, it is possible to suppress the static pressure dependence of the pump current of the adjustment pump unit from being too high. The inventors have confirmed this effect through experiments, analysis, etc. As a result, it is possible to provide a sensor element that suppresses the static pressure dependence of the pump current of the adjustment pump unit from being too high. It should be noted that in the case of manufacturing the element body by stacking a plurality of solid electrolyte layers and further sintering them to achieve integration, in most cases, the central portion of the diffusion rate control portion in the width direction bulges or is recessed relative to its two end portions, and the height of the diffusion rate control portion is uneven at each position in the width direction. Therefore, it is difficult to actually measure the height of the diffusion rate control portion. In contrast, by using the above-mentioned formula (A), the height t (average height) of the diffusion rate control portion can be calculated.

[0018] [2] In the sensor element of the present invention (the sensor element described in [1] above), the height t may be 0.0090 or greater. This can further suppress excessive static pressure dependence of the pump current of the adjustment pump unit.

[0019] [3] In the sensor element of the present invention (the sensor element described in [1] or [2] above), the height t may be 0.0250 or less. This can suppress the degradation rate of the adjustment pump unit. The inventors have confirmed this effect through experiments, analysis, and the like.

[0020] [4] In the sensor element of the present invention (the sensor element described in any of [1] to [3] above), the diffusion rate control section may include first to nth (n ≥ 2) diffusion rate control sections, L / H is obtained by using the path length Li (i: 1 to n) [cm] and width Hi [cm] of each of the first to nth diffusion rate control sections and summing Li / Hi, and the height t is the average of the heights ti of the first to nth diffusion rate control sections. Here, the height ti of the i-th diffusion rate control section corresponds to the height of a single slit when the i-th diffusion rate control section includes only one slit having a path length Li and a width Hi, and corresponds to the total height of the plurality of slits when the i-th diffusion rate control section includes multiple slits having path lengths Li and widths Hi.

[0021] [5] In the sensor element of the present invention (the sensor element described in any one of [1] to [4] above), the plurality of oxygen concentration adjustment chambers and the adjustment pump unit having the inner electrode may be connected in series along the measured gas flow section, the diffusion rate control unit may be provided upstream of the oxygen concentration adjustment chamber on the most upstream side in the measured gas flow section, and the limiting current Ip may be the limiting current of the adjustment pump unit that adjusts the oxygen concentration in the oxygen concentration adjustment chamber on the most upstream side in the measured gas flow section. In this case, the static pressure dependence of the pump current of the oxygen concentration adjustment chamber on the most upstream side in the measured gas flow section can be suppressed.

[0022] [6] The gas sensor of the present invention is primarily characterized by comprising the sensor element described in any one of [1] to [5]. Therefore, the gas sensor of the present invention can achieve the same effects as the sensor element of the present invention described above, for example, providing a sensor element that suppresses excessive static pressure dependence of the pump current of the adjustment pump unit.

[0023] [7] The main points of the sensor element evaluation method of the present invention are:

[0024] A method for evaluating a sensor element for detecting a specific gas concentration in a measured gas.

[0025] The sensor element comprises:

[0026] an element body having an oxygen ion conductive solid electrolyte layer and provided with a gas flow portion therein for introducing and flowing the gas to be measured;

[0027] an adjustment pump unit having an inner electrode disposed in the oxygen concentration adjustment chamber in the measured gas flow portion and adjusting the oxygen concentration in the oxygen concentration adjustment chamber;

[0028] a measuring electrode disposed in a measuring chamber downstream of the oxygen concentration adjustment chamber in the measured gas flow portion; and

[0029] a diffusion rate control unit provided in the measured gas flow unit, which applies diffusion resistance to the measured gas from the outside and introduces the measured gas into the oxygen concentration adjustment chamber;

[0030] The evaluation method performs the following steps on the sensor element to be evaluated:

[0031] (a) The path length L [cm] of the diffusion rate control section, the width H [cm] of the diffusion rate control section, the limiting current Ip [A] of the adjustment pump unit, the Faraday constant F [A·sec / mol], the diffusion coefficient D [cm] of oxygen were used. 2 / sec], gas constant R[cm 3 ·atm / mol·K], the temperature T[K] of the inner electrode, the oxygen partial pressure Poe[atm] in the measured gas, and the oxygen partial pressure Pod[atm] in the oxygen concentration adjustment chamber, and using formula (B), calculate the height t[mm] of the diffusion rate control unit; and

[0032] (b) Evaluation is performed using the height t.

[0033] t=L / H×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10(B)

[0034] In the sensor element evaluation method of the present invention, the path length L [cm] of the diffusion rate control portion, the width H [cm] of the diffusion rate control portion, the Faraday constant F [A·sec / mol], the oxygen diffusion coefficient D [cm], and the diffusion rate of the sensor element to be evaluated are used. 2 / sec], gas constant R[cm 3 ·atm / mol·K], the temperature of the inner electrode T[K], the limiting current Ip[A] of the adjustment pump unit, the oxygen partial pressure Poe[atm] in the measured gas, and the oxygen partial pressure Pod[atm] in the oxygen concentration adjustment chamber are used to calculate the height t[mm] of the diffusion rate control unit using formula (B). The calculated height t[mm] is used for evaluation. In this way, the product of the width H[cm] and the height t / 10[cm] of the diffusion rate control unit, i.e., the cross-sectional area [cm], can be evaluated. 2 ] is too small; in other words, whether the diffusion resistance from the outside to the oxygen concentration adjustment chamber is too high. Therefore, it is possible to evaluate whether the static pressure dependence of the pump current of the adjustment pump unit is too high. The inventors have confirmed this effect through experiments and analysis. As a result, it is possible to provide a sensor element that suppresses the excessive static pressure dependence of the pump current of the adjustment pump unit.

[0035] [8] The purpose of the program of the present invention is to enable one or more computers to execute the various steps of the sensor element evaluation method of the present invention (the sensor element evaluation method described in [7] above). The program can be recorded on a computer-readable recording medium (such as a hard disk, SSD, ROM, FD, CD, DVD, etc.), or can be sent from one computer to another via a transmission medium (communication network such as the Internet and LAN), or can be transferred in other forms. If one or more computers execute the program of the present invention, the various steps of the sensor element evaluation method of the present invention can be executed, and thus, the same effects as those of the sensor element evaluation method of the present invention can be obtained, such as the effect of providing a sensor element that suppresses the excessive static pressure dependence of the pump current of the adjustment pump unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 1 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100 .

[0037] Figure 2 It is an enlarged view of the first and second diffusion rate control units 11 and 13 and their surroundings of the gas sensor 100 .

[0038] Figure 3 It is a block diagram showing the electrical connection relationship between the control device 95 and each unit.

[0039] Figure 4 2 is a schematic cross-sectional view of a sensor element 201 according to a modified example.

[0040] Figure 5 is a graph showing the relationship between the average height t and the static pressure dependency index α. DETAILED DESCRIPTION

[0041] Next, embodiments of the present invention will be described with reference to the accompanying drawings. Figure 1 1 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor 100 as one embodiment of the present invention. Figure 2 It is an enlarged view of the first and second diffusion rate control units 11 and 13 and their surroundings of the gas sensor 100 . Figure 3: This is a block diagram showing the electrical connection relationship between the control device 95 and each unit and the heater 72. The gas sensor 100 is installed in a pipe such as an exhaust pipe of an internal combustion engine. The gas sensor 100 uses the exhaust gas of the internal combustion engine as the measured gas and detects the concentration of specific gases such as NOx and ammonia in the measured gas, that is, the specific gas concentration. In this embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The gas sensor 100 includes: a sensor element 101, which has an element body 102 in the shape of an elongated rectangular parallelepiped; each unit 21, 41, 50, 80 to 83 of the sensor element 101 (element body 102); a heater unit 70, which is provided inside the sensor element 101; and a control device 95, which has variable power supplies 24, 46, 52 and a heater power supply 76 and controls the entire gas sensor 100.

[0042] The sensor element 101 (element body 102) comprises a laminated body composed, starting from the bottom in the figure, of six layers: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a separator layer 5, and a second solid electrolyte layer 6, each composed of an oxygen ion conductive solid electrolyte layer such as zirconium oxide (ZrO2). The solid electrolytes forming these six layers are dense, airtight solid electrolytes. The element body 102 is manufactured by, for example, subjecting ceramic green sheets corresponding to each layer to predetermined processing and printing circuit patterns, then laminating and sintering them to form a single unit.

[0043] On the front end side of the sensor element 101 (element body 102) ( Figure 1 On the left end side) and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas inlet 10, a first diffusion rate control part 11, a buffer space 12, a second diffusion rate control part 13, a first internal cavity (oxygen concentration adjustment chamber) 20, a third diffusion rate control part 30, a second internal cavity (oxygen concentration adjustment chamber) 40, a fourth diffusion rate control part 60 and a third internal cavity (measurement chamber) 61 are adjacently formed in a sequentially connected manner.

[0044] The gas inlet 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40 and the third internal cavity 61 are spaces inside the sensor element 101 arranged in a manner of hollowing out the isolation layer 5, wherein their upper parts are separated by the lower surface of the second solid electrolyte layer 6, their lower parts are separated by the upper surface of the first solid electrolyte layer 4, and their sides are separated by the side surfaces of the isolation layer 5.

[0045] The first diffusion rate control section 11, the second diffusion rate control section 13, and the third diffusion rate control section 30 are each configured as two horizontally long slits (in the direction perpendicular to the drawing, forming the length of the opening). Furthermore, the fourth diffusion rate control section 60 is configured as a single horizontally long slit (in the direction perpendicular to the drawing, forming the length of the opening) formed as a gap with the lower surface of the second solid electrolyte layer 6. The portion from the gas inlet 10 to the third internal cavity 61 is referred to as the measured gas flow section.

[0046] The sensor element 101 (element body 102) includes a reference gas inlet portion 49 that allows a reference gas used for NOx concentration measurement to flow from the outside of the sensor element 101 to the reference electrode 42. The reference gas inlet portion 49 comprises a reference gas inlet space 43 and a reference gas inlet layer 48. The reference gas inlet space 43 extends inward from the rear end face of the sensor element 101. The reference gas inlet space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the separator layer 5, at a position separated from the side face of the first solid electrolyte layer 4. The reference gas inlet space 43 opens at the rear end face of the sensor element 101, and this opening serves as an inlet portion 49a of the reference gas inlet portion 49. The reference gas is introduced into the reference gas inlet space 43 through this inlet portion 49a. The reference gas is introduced into the reference gas inlet space 43 by applying a predetermined diffusion resistance to the reference gas introduced through the inlet portion 49a, and then is introduced to the reference electrode 42. In this embodiment, the reference gas is atmospheric air.

[0047] Reference gas introduction layer 48 is provided between the upper surface of third substrate layer 3 and the lower surface of first solid electrolyte layer 4. Reference gas introduction layer 48 is a porous body composed of a ceramic such as alumina. A portion of the upper surface of reference gas introduction layer 48 is exposed within reference gas introduction space 43. Reference gas introduction layer 48 is formed to cover reference electrode 42. Reference gas introduction layer 48 allows reference gas to flow from reference gas introduction space 43 to reference electrode 42.

[0048] The reference electrode 42 is formed so as to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, a reference gas introduction layer 48 connected to the reference gas introduction space 43 is provided around the reference electrode 42. As will be described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) within the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61.

[0049] In the measured gas flow section, the gas inlet 10 is a portion open to the outside space, and the measured gas enters the sensor element 101 from the outside space through the gas inlet 10. The first diffusion rate control section 11 is a portion that applies a predetermined diffusion resistance to the measured gas entering from the gas inlet 10. The first diffusion rate control section 11 is as follows: Figure 2 The apparatus shown has an upper slit 11a and a lower slit 11b. The upper slit 11a is formed as a horizontally long slit in the vertical direction between the upper surface of the partition wall 5a, which is part of the isolation layer 5, and the lower surface of the second solid electrolyte layer 6. The lower slit 11b is formed as a horizontally long slit between the lower surface of the partition wall 5a and the upper surface of the first solid electrolyte layer 4. The partition wall 5a forms the portion between the outside and the buffer space 12. The left and right sides of the partition wall 5a are connected to other parts of the isolation layer 5, and there is no gap on the left and right sides of the partition wall 5a for the gas to be measured to flow. In this embodiment, the upper slit 11a and the lower slit 11b are formed so that the path length, that is, the length in the front-to-back direction, is the same as the path length L1, and the width, that is, the length in the lateral direction, is the same as the width H1.

[0050] Return to Figure 1 The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate control unit 11 to the second diffusion rate control unit 13. The second diffusion rate control unit 13 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 to the first internal cavity 20. Figure 2 As shown, the second diffusion rate control section 13 includes an upper slit 13a and a lower slit 13b. The upper slit 13a is formed as a horizontally long slit between the upper surface of the partition wall 5b, which is part of the separator layer 5, and the lower surface of the second solid electrolyte layer 6. The lower slit 13b is formed as a horizontally long slit in the vertical direction between the lower surface of the partition wall 5b and the upper surface of the first solid electrolyte layer 4. The partition wall 5b forms the portion between the buffer space 12 and the first internal cavity 20. The left and right sides of the partition wall 5b are connected to other parts of the separator layer 5, and there is no gap on the left and right sides of the partition wall 5b for the gas to be measured to flow. In this embodiment, the upper slit 13a and the lower slit 13b are formed so that the path length, i.e., the length in the front-to-back direction, is the same as the path length L2, and the width, i.e., the length in the lateral direction, is the same as the width H2.

[0051] When the measured gas is introduced from outside the sensor element 101 into the first internal cavity 20, the measured gas that rapidly enters the sensor element 101 from the gas inlet 10 due to pressure fluctuations in the external space (in the case of automobile exhaust, this is exhaust pressure pulsation) is not introduced directly into the first internal cavity 20. Instead, it is introduced into the first internal cavity 20 after the pressure fluctuations of the measured gas are eliminated by the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13. As a result, the pressure fluctuations of the measured gas introduced into the first internal cavity 20 are almost negligible. The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the measured gas introduced through the second diffusion rate control unit 13. This oxygen partial pressure is adjusted by the operation of the main pump unit 21.

[0052] The main pump unit 21 is an electrochemical pump unit composed of an inner pump electrode 22, an outer pump electrode 23, a second solid electrolyte layer 6, an isolation layer 5, and a first solid electrolyte layer 4 that serve as a current path between these electrodes. The inner pump electrode 22 has a top electrode portion 22a that is arranged on substantially the entire surface of the lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, and the outer pump electrode 23 is arranged in an area corresponding to the top electrode portion 22a on the upper surface of the second solid electrolyte layer 6 in a manner exposed to the outside of the sensor element 101.

[0053] The inner pump electrode 22 spans the solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) formed above and below the first internal cavity 20, and the spacer layer 5 forming the sidewalls. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 forming the top surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 forming the bottom surface of the first internal cavity 20. Furthermore, side electrodes (not shown) are formed on the sidewall surfaces (inner surfaces) of the spacer layer 5 forming the two sidewalls of the first internal cavity 20, connecting the top electrode portion 22a and the bottom electrode portion 22b. Consequently, the side electrodes are arranged in a tunnel-like structure at the locations where they are arranged.

[0054] In the main pump unit 21, the desired voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, so that the pump current Ip0 flows between the inner pump electrode 22 and the outer pump electrode 23 in the positive direction or the negative direction, thereby being able to absorb the oxygen in the first internal cavity 20 into the external space, or to absorb the oxygen in the external space into the first internal cavity 20.

[0055] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, an electrochemical sensor unit, i.e., an oxygen partial pressure detection sensor unit 80 for main pump control, is formed by the inner pump electrode 22, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42.

[0056] By measuring the electromotive force (voltage V0) of the main pump control oxygen partial pressure detection sensor unit 80, the oxygen concentration (oxygen partial pressure) within the first internal cavity 20 can be determined. Furthermore, the pump current Ip0 is controlled by feedback controlling the voltage Vp0 of the variable power supply 24 so that the voltage V0 reaches a target value. This maintains the oxygen concentration within the first internal cavity 20 at a predetermined constant value.

[0057] The third diffusion rate control unit 30 applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump unit 21 in the first internal cavity 20 , and guides the gas to be measured to the second internal cavity 40 .

[0058] The second internal cavity 40 is provided as a space for performing the following process: the oxygen concentration (oxygen partial pressure) is preliminarily adjusted in the first internal cavity 20, and then the oxygen partial pressure of the measured gas introduced through the third diffusion rate control unit 30 is further adjusted using the auxiliary pump unit 50. This allows the oxygen concentration in the second internal cavity 40 to be maintained constant with high accuracy, thereby enabling highly accurate NOx concentration measurement in the gas sensor 100.

[0059] The auxiliary pump unit 50 is an auxiliary electrochemical pump unit composed of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is an appropriate electrode outside the sensor element 101), a second solid electrolyte layer 6, an isolation layer 5, and a first solid electrolyte layer 4. The auxiliary pump electrode 51 has a top electrode portion 51a that is roughly entirely arranged on the lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40.

[0060] The auxiliary pump electrode 51 is disposed within the second internal cavity 40 in a tunnel-shaped structure similar to the inner pump electrode 22 disposed within the first internal cavity 20. Specifically, a top electrode portion 51 a is formed relative to the second solid electrolyte layer 6 forming the top surface of the second internal cavity 40, and a bottom electrode portion 51 b is formed on the first solid electrolyte layer 4 forming the bottom surface of the second internal cavity 40. Furthermore, a side electrode portion (not shown) connecting the top electrode portion 51 a and the bottom electrode portion 51 b is formed in a tunnel-shaped structure formed on both wall surfaces of the isolation layer 5 forming the sidewalls of the second internal cavity 40.

[0061] In the auxiliary pump cell 50 , a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 , thereby absorbing oxygen in the atmosphere within the second internal cavity 40 to the external space or absorbing oxygen from the external space into the second internal cavity 40 .

[0062] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an electrochemical sensor unit, namely, an auxiliary pump control oxygen partial pressure detection sensor unit 81, is composed of an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, and a third substrate layer 3.

[0063] Furthermore, the auxiliary pump unit 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor unit 81. As a result, the oxygen partial pressure in the atmosphere within the second internal cavity 40 is controlled to a low partial pressure that has substantially no effect on NOx measurement.

[0064] Simultaneously, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor unit 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor unit 80. By controlling the target value of its voltage V0, the oxygen partial pressure gradient in the gas to be measured, introduced from the third diffusion rate control unit 30 into the second internal cavity 40, is controlled to remain constant. When used as a NOx sensor, the oxygen concentration within the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm by the main pump unit 21 and the auxiliary pump unit 50.

[0065] The fourth diffusion rate control unit 60 applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump unit 50 in the second internal cavity 40, and introduces the gas to be measured into the third internal cavity 61. The fourth diffusion rate control unit 60 has the function of limiting the amount of NOx flowing into the third internal cavity 61.

[0066] The third internal cavity 61 is configured as a space for performing processing related to the measurement of nitrogen oxide (NOx) concentration in the gas to be measured, which is introduced through the fourth diffusion rate control unit 60, after the oxygen concentration (oxygen partial pressure) is pre-adjusted in the second internal cavity 40. NOx concentration measurement is primarily performed in the third internal cavity 61 by operating the measurement pump unit 41.

[0067] The measurement pump cell 41 measures the NOx concentration in the gas being measured within the third internal cavity 61. The measurement pump cell 41 is an electrochemical pump cell consisting of a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a separator 5, and a first solid electrolyte layer 4. The measurement electrode 44 is located on the upper surface of the first solid electrolyte layer 4, facing the third internal cavity 61. The measurement electrode 44 also functions as a NOx reduction catalyst, reducing NOx in the atmosphere within the third internal cavity 61.

[0068] The measurement pump cell 41 can absorb oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 , and the generated amount can be detected as the pump current Ip2 .

[0069] Furthermore, to detect the oxygen partial pressure around the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 constitute an electrochemical sensor unit, namely, a measuring pump control oxygen partial pressure detection sensor unit 82. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the measuring pump control oxygen partial pressure detection sensor unit 82.

[0070] The gas to be measured, introduced into the second internal cavity 40, passes through the fourth diffusion rate control unit 60 while the oxygen partial pressure is controlled, and reaches the measuring electrode 44 within the third internal cavity 61. Nitrogen oxides in the gas to be measured are reduced (2NO → N2 + O2), generating oxygen. This generated oxygen is then pumped by the measurement pump unit 41. During this process, the voltage Vp2 of the variable power supply 46 is controlled to maintain a constant voltage (target value) as detected by the oxygen partial pressure detection sensor unit 82 for measuring pump control. The amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured. Therefore, the nitrogen oxide concentration in the gas to be measured is calculated using the pump current Ip2 of the measurement pump unit 41.

[0071] In addition, if the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detection mechanism in the form of an electrochemical sensor unit, the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere can be detected, thereby also being able to determine the concentration of the NOx component in the measured gas.

[0072] In addition, the electrochemical sensor unit 83 is composed of the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42. The electromotive force (voltage Vref) obtained by the sensor unit 83 can be used to detect the oxygen partial pressure in the measured gas outside the sensor.

[0073] In the gas sensor 100 having such a configuration, the main pump cell 21 and the auxiliary pump cell 50 are operated to supply the measurement pump cell 41 with the gas to be measured, whose oxygen partial pressure is always maintained at a constant low value (a value that has substantially no effect on NOx measurement). Therefore, the NOx concentration in the gas to be measured can be determined based on the pump current Ip2 that is substantially proportional to the NOx concentration in the gas to be measured and flows as oxygen generated by NOx reduction is drawn from the measurement pump cell 41.

[0074] Here, each of electrodes 22, 23, 42, 44, and 51 is described. The inner pump electrode 22, auxiliary pump electrode 51, and measuring electrode 44 each contain a first noble metal having catalytic activity. Examples of the first noble metal include at least one of Pt, Rh, Ir, Ru, and Pd. The outer pump electrode 23 and reference electrode 42 also contain the first noble metal. The inner pump electrode 22 and auxiliary pump electrode 51 also contain a second noble metal that suppresses the catalytic activity of the first noble metal toward a specific gas (NOx). This weakens the ability of the inner pump electrode 22 and auxiliary pump electrode 51 to reduce NOx components in the measured gas. An example of the second noble metal is Au. The measuring electrode 44 does not contain the second noble metal. As a result, its ability to reduce NOx components in the measured gas is improved compared to the inner pump electrode 22 and auxiliary pump electrode 51. It is also preferable that the outer pump electrode 23 and reference electrode 42 do not contain the second noble metal. Each electrode 22, 23, 42, 44, and 51 is preferably a cermet containing a noble metal and an oxide with oxygen ion conductivity (e.g., ZrO2). Each electrode 22, 23, 42, 44, and 51 is preferably a porous body. In this embodiment, the inner pump electrode 22 and the auxiliary pump electrode 51 are porous cermet electrodes made of Pt containing 1% Au and ZrO2. Furthermore, the outer pump electrode 23, the reference electrode 42, and the measuring electrode 44 are all porous cermet electrodes made of Pt and ZrO2.

[0075] Sensor element 101 includes a heater unit 70, which heats and maintains the sensor element 101 to improve the oxygen ion conductivity of the solid electrolyte. Heater unit 70 includes a heater connector electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0076] The heater connector electrode 71 is an electrode formed in contact with the lower surface of the first substrate layer 1. Figure 3 ) is connected, and power can be supplied from the heater power supply 76 to the heater unit 70.

[0077] Heater 72 is a resistor formed so as to be sandwiched from above and below by second substrate layer 2 and third substrate layer 3. Heater 72 is connected to heater connector electrode 71 via through-hole 73. Heater 72 is generated by power supplied from heater power supply 76 via heater connector electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming sensor element 101.

[0078] Furthermore, the heater 72 is embedded in the entire region from the first internal cavity 20 to the third internal cavity 61 , and can adjust the entire sensor element 101 to a temperature that activates the solid electrolyte.

[0079] Heater insulating layer 74 is formed of an insulator such as alumina on the upper and lower surfaces of heater 72. Heater insulating layer 74 is formed to achieve electrical insulation between second substrate layer 2 and heater 72, and between third substrate layer 3 and heater 72.

[0080] The pressure release hole 75 is provided to penetrate the third substrate layer 3 and the reference gas introduction layer 48 and communicate with the reference gas introduction space 43 , and is formed to alleviate the internal pressure increase associated with the temperature increase in the heater insulation layer 74 .

[0081] like Figure 3As shown, the control device 95 includes the variable power supplies 24, 46, and 52 described above, the heater power supply 76 described above, and a control unit 96. The control unit 96 is a microprocessor including a CPU 97 and a storage unit 98. The storage unit 98 is a rewritable nonvolatile memory capable of storing, for example, various programs and data. Inputs to the control unit 96 are the voltage V0 of the main pump control oxygen partial pressure detection sensor cell 80, the voltage V1 of the auxiliary pump control oxygen partial pressure detection sensor cell 81, the voltage V2 of the measurement pump control oxygen partial pressure detection sensor cell 82, the voltage Vref of the sensor cell 83, the pump current Ip0 flowing through the main pump cell 21, the pump current Ip1 flowing through the auxiliary pump cell 50, and the pump current Ip2 flowing through the measurement pump cell 41. The control unit 96 also outputs control signals to the variable power supplies 24, 46, and 52, thereby controlling the voltages Vp0, Vp1, and Vp2 output by the variable power supplies 24, 46, and 52, and thereby controlling the main pump unit 21, the measurement pump unit 41, and the auxiliary pump unit 50. The control unit 96 also outputs control signals to the heater power supply 76, thereby controlling the power supplied by the heater power supply 76 to the heater 72. The storage unit 98 also stores target values V0*, V1*, and V2*, which will be described later. The CPU 97 of the control unit 96 controls each unit 21, 41, and 50 by referring to these target values V0*, V1*, and V2*.

[0082] The control unit 96 performs auxiliary pump control processing to control the auxiliary pump unit 50 so that the oxygen concentration in the second internal cavity 40 reaches a target concentration. Specifically, the control unit 96 controls the auxiliary pump unit 50 by feedback controlling the voltage Vp1 of the variable power supply 52 so that the voltage V1 reaches a constant value (referred to as the target value V1*). The target value V1* is determined so that the oxygen concentration in the second internal cavity 40 reaches a predetermined low concentration that has no substantial effect on NOx measurement.

[0083] The control unit 96 performs the following main pump control process: it controls the main pump unit 21 so that the pump current Ip1 flowing through the auxiliary pump unit 50 when adjusting the oxygen concentration in the second internal cavity 40 through the auxiliary pump control process reaches a target current (referred to as target value Ip1*). Specifically, the control unit 96 sets a target value for voltage V0 (referred to as target value V0*) based on the pump current Ip1 (feedback control) so that the pump current Ip1 flowing due to voltage Vp1 reaches a constant target value Ip1*. Furthermore, the control unit 96 performs feedback control on voltage Vp0 of the variable power supply 24 so that voltage V0 reaches the target value V0* (i.e., so that the oxygen concentration in the first internal cavity 20 reaches the target concentration). This main pump control process maintains a constant gradient of the oxygen partial pressure in the gas to be measured introduced from the third diffusion rate control unit 30 into the second internal cavity 40. The target value V0* is set to a value such that the oxygen concentration in the first internal cavity 20 is higher than 0% and low. Furthermore, the pump current Ip0 flowing during this main pump control process varies depending on the oxygen concentration of the gas to be measured (i.e., the gas to be measured surrounding sensor element 101) flowing from gas inlet 10 into the gas to be measured flow portion. Therefore, control unit 96 can also detect the oxygen concentration in the gas to be measured based on pump current Ip0.

[0084] The main pump control process and the auxiliary pump control process described above are also collectively referred to as the adjustment pump control process. Furthermore, the first internal cavity 20 and the second internal cavity 40 are also collectively referred to as the oxygen concentration adjustment chamber. The main pump unit 21 and the auxiliary pump unit 50 are also collectively referred to as the adjustment pump unit. The adjustment pump control process is performed by the control unit 96, whereby the adjustment pump unit adjusts the oxygen concentration in the oxygen concentration adjustment chamber.

[0085] The control unit 96 also performs measurement pump control processing to control the measurement pump cell 41 so that the voltage V2 reaches a constant value (referred to as the target value V2*) (that is, so that the oxygen concentration in the third internal cavity 61 reaches a predetermined low concentration). Specifically, the control unit 96 controls the measurement pump cell 41 by feedback controlling the voltage Vp2 of the variable power supply 46 so that the voltage V2 reaches the target value V2*. This measurement pump control processing allows oxygen to be drawn from the third internal cavity 61.

[0086] By performing measurement pump control processing, oxygen is drawn from the third internal cavity 61 so that the amount of oxygen generated by reducing NOx in the measured gas within the third internal cavity 61 is substantially zero. Furthermore, the control unit 96 obtains a pump current Ip2 as a detection value corresponding to the oxygen generated within the third internal cavity 61 from the specific gas (here, NOx), and calculates the NOx concentration in the measured gas based on this pump current Ip2.

[0087] The storage unit 98 stores a relational expression (for example, a linear function or a quadratic function), a map, etc. as a correspondence between the pump current Ip2 and the NOx concentration. The relational expression or the map can be solved in advance through experiments.

[0088] The control unit 96 performs heater control processing by outputting a control signal to the heater power supply 76 to control the temperature of the heater 72 so that it reaches a target temperature (e.g., 800°C). The temperature of the heater 72 can be expressed as a linear function of the resistance value of the heater 72. Therefore, during the heater control processing, the control unit 96 calculates the resistance value of the heater 72 as a value that can be regarded as the temperature of the heater 72 (a value that can be converted into temperature), and performs feedback control on the heater power supply 76 so that the calculated resistance value reaches the target resistance value (the resistance value corresponding to the target temperature). The control unit 96 can obtain, for example, the voltage of the heater 72 and the current flowing through the heater 72, and calculate the resistance value of the heater 72 based on the obtained voltage and current. The control unit 96 can calculate the resistance value of the heater 72 using, for example, a three-terminal method or a four-terminal method. When the heater power supply 76 supplies power to the heater 72, for example, it changes the value of the voltage applied to the heater 72 based on the control signal from the control unit 96, thereby adjusting the power supplied to the heater 72.

[0089] In addition, including Figure 3 The control device 95 controls the sensor element 101 by means of the variable power supplies 24, 46, 52 and the heater power supply 76 shown in the figure, and the unshown lead wires formed in the sensor element 101 and the unshown connector electrodes formed on the rear end side of the sensor element 101 (only the heater connector electrode 71 is shown in the figure). Figure 1 ) and connected to the electrodes inside the sensor element 101.

[0090] In addition, regarding the element body 102 of the sensor element 101, as shown in FIG. Figure 1As shown, the front end portion is covered with a porous protective layer 77. The porous protective layer 77 covers a portion of the upper and lower surfaces of the element body 102. Although not shown, the porous protective layer 77 also covers a portion of the left and right surfaces of the element body 102. The porous protective layer 77 also covers the front surface of the element body 102. The porous protective layer 77 also covers the outer pump electrode 23. The porous protective layer 77 also covers the gas inlet 10. Because the porous protective layer 77 is porous, the measured gas can flow through the porous protective layer 77 and reach the outer pump electrode 23 and the gas inlet 10. The porous protective layer 77 covers a portion of the element body 102 and protects it. The porous protective layer 77 plays a role in suppressing cracks in the element body 102 caused by the adhesion of moisture in the measured gas. The porous protective layer 77 also plays a role in preventing poisoning substances such as oil contained in the measured gas from adhering to the outer pump electrode 23, thereby suppressing degradation of the outer pump electrode 23. The porous protective layer 77 is a porous body composed of a ceramic such as alumina. While not particularly limited, the porosity of the porous protective layer 77 is preferably, for example, 10% to 60%, and the thickness t of the porous protective layer 77 (the length in the front-to-back direction of the portion covering the front end of the element body 102) is preferably, for example, 300 μm to 700 μm.

[0091] It should be noted that the porosity of the porous protective layer 77 is set to a value derived as follows from an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the object to be measured is cut in such a manner that the cross section of the object to be measured is set as the observation surface, and the cross section is resin-filled and polished to prepare an observation sample. Next, the observation surface of the observation sample is photographed with an SEM photograph (secondary electron image, acceleration voltage 15kV, magnification 1000 times, wherein, when the magnification 1000 times is inappropriate, a magnification greater than 1000 times and less than 5000 times is used), thereby obtaining an SEM image of the object to be measured. Next, the obtained image is subjected to image analysis, and a threshold is determined using a discriminant analysis method (Otsu's binarization) based on the brightness distribution of the brightness data of the pixels in the image. Thereafter, based on the determined threshold, each pixel in the image is binarized into an object portion and a pore portion, and the area of the object portion and the area of the pore portion are calculated. Then, the ratio of the area of the pore portion to the total area (the total area of the object portion and the pore portion) was derived as the porosity (unit: %).

[0092] Next, an example of a method for manufacturing the sensor element 101 of the gas sensor 100 is described below. First, six unfired ceramic green sheets containing an oxygen ion conductive solid electrolyte such as zirconium oxide as a ceramic component are prepared. A plurality of sheet holes and necessary through-holes are pre-formed in these green sheets for positioning during printing and lamination. In addition, a space for the gas to be measured is pre-set in the green sheet constituting the separator 5 by punching or the like. A space for the reference gas introduction space 43 is also pre-set in the green sheet constituting the first solid electrolyte layer 4. Then, a pattern printing process is performed to form various patterns corresponding to the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the separator 5, and the second solid electrolyte layer 6, and a drying process is performed. Specifically, the patterns formed are, for example, patterns of the aforementioned electrodes, the leads connected to the electrodes, the reference gas introduction layer 48, the heater portion 70, and the like. The pattern printing is performed as follows: a pattern forming paste prepared according to the characteristics required for each object to be formed is applied to a raw sheet using a known screen printing technique. The drying process is also performed using a known drying method. When the pattern printing and drying are completed, a bonding paste is printed and dried for stacking and bonding the raw sheets corresponding to each layer. Then, a pressing process is performed, that is, the raw sheets formed with the bonding paste are positioned using the sheet holes and stacked in a specified order, and specified temperature and pressure conditions are applied to press them together to form a stacked body. The stacked body thus obtained contains a plurality of component bodies 102. The stacked body is cut and divided into the size of the component body 102. Then, the divided stacked body is fired at a specified firing temperature to obtain the component body 102.

[0093] Next, a porous protective layer 77 is formed on the element body 102 to obtain the sensor element 101. The porous protective layer 77 can be formed using at least one of plasma deposition, screen printing, gel casting, and dipping. When the porous protective layer 77 is formed by screen printing, dipping, or other methods accompanied by firing, the pre-fired porous protective layer 77 can be formed on the pre-fired element body 102 and both can be fired simultaneously to obtain the sensor element 101. After the sensor element 101 is obtained in this way, it is housed in a predetermined housing and embedded in the gas sensor body (not shown) to obtain the gas sensor 100.

[0094] Here, the fourth diffusion rate control section 60 can be formed, for example, as follows. First, in the above-mentioned pattern printing process, a vanishing material (e.g., theobromine) that disappears upon firing is pre-applied to the upper surface of the portion of the green sheet constituting the isolation layer 5 that will serve as the partition wall. Thus, during the above-mentioned firing, the vanishing material disappears, and a gap (a horizontally long slit) is formed between the upper surface of the partition wall in the isolation layer 5 and the lower surface of the second solid electrolyte layer 6, forming the fourth diffusion rate control section 60. It should be noted that the vanishing material can be applied not only to the upper surface of the portion that will serve as the partition wall, but also to the portion of the lower surface of the green sheet constituting the second solid electrolyte layer 6 that faces the partition wall. Furthermore, by adjusting the coating thickness of the vanishing material, the upper and lower heights of the slit in the fourth diffusion rate control section 60 can be adjusted. The first to third diffusion rate control sections 11, 13, and 30 can be formed in the same manner, except that the vanishing material is pre-applied to the upper and lower surfaces of the isolation layer 5. In addition, the method of forming such a diffusion rate control portion is publicly known and is described in, for example, Japanese Patent No. 4911910.

[0095] Next, an example of using the gas sensor 100 will be described. The CPU 97 of the control device 95 is configured to control the aforementioned pump units 21, 41, and 50 (adjustment pump control processing and measurement pump control processing), and to obtain the voltages V0, V1, V2, and Vref from the aforementioned sensor units 80-83. In this state, when the measured gas is introduced into the element body 102 via the gas inlet 10, the measured gas first passes through the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13 in sequence before reaching the first internal cavity 20. Next, the oxygen concentration of the measured gas is adjusted within the first internal cavity 20 and the second internal cavity 40 by the main pump unit 21 and the auxiliary pump unit 50, and the adjusted measured gas reaches the third internal cavity 61. Furthermore, the CPU 97 obtains the pump current Ip2 and, based on the obtained pump current Ip2, detects the NOx concentration in the measured gas.

[0096] In this embodiment, the sensor element 101 is configured such that the average height t of the first and second diffusion rate control sections 11 and 13 is greater than or equal to 0.0035 mm. Preferably, the sensor element 101 is configured such that the average height t of the first and second diffusion rate control sections 11 and 13 is greater than or equal to 0.0090 mm. Furthermore, preferably, the sensor element 101 is configured such that the average height t of the first and second diffusion rate control sections 11 and 13 is less than or equal to 0.0250 mm. The average height t of the first and second diffusion rate control sections 11 and 13 corresponds to the average of the heights t1 and t2 of the diffusion rate control sections 11 and 13. The height t1 of the first diffusion rate control section 11 corresponds to the sum of the height t11 of the upper slit 11a and the height t12 of the lower slit 11b of the first diffusion rate control section 11. The height t2 of the second diffusion rate control section 13 corresponds to the sum of the height t21 of the upper slit 13a and the height t22 of the lower slit 13b of the second diffusion rate control section 13.

[0097] The average height t of the first and second diffusion rate control sections 11 and 13 is calculated using the path lengths L1 and L2 [cm] of the first and second diffusion rate control sections 11 and 13, the widths H1 and H2 [cm] of the first and second diffusion rate control sections 11 and 13, the limiting current Ip [A] of the main pump unit 21, the Faraday constant F [A·sec / mol], the diffusion coefficient D [cm] of oxygen, and the average height t of the first and second diffusion rate control sections 11 and 13. 2 / sec], gas constant R[cm 3 =·atm / mol·K], the temperature T[K] of the inner pump electrode 22, the oxygen partial pressure Poe[atm] in the measured gas, and the oxygen partial pressure Pod[atm] in the first internal cavity 20 are obtained using equation (1). The path lengths L1 and L2 of the first and second diffusion rate control sections 11 and 13 are the path lengths of the upper slits 11a and 13a and the lower slits 11b and 13b, respectively, that is, the lengths in the front-to-back direction. The widths H1 and H2 of the first and second diffusion rate control sections 11 and 13 are the widths of the upper slits 11a and 13a and the lower slits 11b and 13b, respectively, that is, the lengths in the left-to-right direction. It should be noted that, when the upper slits 11a, 13a and the lower slits 11b, 13b of the first and second diffusion rate control sections 11, 13 are collectively considered as a single diffusion rate control section, the height t of the diffusion rate control section is obtained using the path length L, width H, oxygen diffusion coefficient D, gas constant R, temperature T of the inner pump electrode 22, oxygen partial pressure Poe in the measured gas, and oxygen partial pressure Pod in the first internal cavity 20, using equation (2). Equation (1) is equivalent to the equation obtained by replacing "L / H" on the right side of equation (2) with "L1 / H1+L2 / H2".

[0098] t=(L1 / H1+L2 / H2)×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10 (1)

[0099] t=L / H×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10 (2)

[0100] In formula (1), the path length L1 of the first diffusion rate control section 11 is 300 μm in this embodiment, and the path length L2 of the second diffusion rate control section 13 is 500 μm in this embodiment. The widths H1 and H2 of the first and second diffusion rate control sections 11 and 13 are both within the range of 1.0 mm to 2.5 mm in this embodiment. The Faraday constant F is 96490 [A·sec / mol] in this embodiment. The diffusion coefficient D of oxygen is 1.6 [cm 2 / sec]. The gas constant R in this embodiment is 82.05 [cm 3 ·atm / mol·K].

[0101] In formula (1), the limiting current Ip of the main pump unit 21 in this embodiment is the limiting current value of the pump current Ip0 when oxygen is sucked out from the periphery of the inner pump electrode 22 toward the periphery of the outer pump electrode 23 when the gas inlet 10 of the sensor element 101 is exposed to an ambient gas with nitrogen as the base gas, an oxygen concentration of 21%, and a pressure of 1 atm.

[0102] The limiting current Ip of the main pump unit 21 can be measured, for example, as follows. First, the gas inlet 10 of the sensor element 101 is exposed to an ambient gas having a nitrogen base gas, an oxygen concentration of 21%, and a pressure of 1 atm. For example, the gas sensor 100 having the sensor element 101 is mounted on the piping in such a manner that the front end portion of the sensor element 101 protrudes into the piping, and the ambient gas is allowed to flow through the piping, thereby exposing the gas inlet 10 of the sensor element 101 to the ambient gas. The oxygen concentration around the reference gas inlet 49 has little effect on the measured value of the limiting current Ip, and the reference gas inlet 49 is exposed to the atmospheric atmosphere. Next, the heater 72 is energized to heat the sensor element 101 to a predetermined drive temperature Tset (e.g., 800°C). At this time, the variable power supplies 24, 46, and 52 are all set to a state where no voltage is applied. After the temperature of sensor element 101 stabilizes, variable power supply 24 applies voltage Vp0 between inner pump electrode 22 and outer pump electrode 23 to draw oxygen from the area around inner pump electrode 22 to the area around outer pump electrode 23. At this point, pump current Ip0 (oxygen draw current) flowing between electrodes 22 and 23 is measured. Voltage Vp0 is set as a DC voltage. As voltage Vp0 is gradually increased, pump current Ip0 also gradually increases. Eventually, even with increasing voltage Vp0, pump current Ip0 stops increasing and reaches its upper limit. The pump current Ip0 at this point is measured as limiting current Ip. The flow rate of gas from the outside, through porous protective layer 77, gas inlet 10, first diffusion rate control section 11, buffer space 12, and second diffusion rate control section 13, reaching first internal cavity 20, i.e., the area around inner pump electrode 22, depends on the diffusion resistance of a predetermined portion of the gas path from the outside to inner pump electrode 22. The diffusion resistance of the specified portion has a negative correlation with the limiting current Ip of the main pump unit 21. Furthermore, the diffusion resistance of the specified portion (limiting current Ip of the main pump unit 21) is particularly influenced by the diffusion resistance of the first and second diffusion rate control units 11 and 13 located upstream of the first internal cavity 20. Furthermore, the diffusion resistance of the first and second diffusion rate control units 11 and 13 is affected by the shapes of the upper slits 11a and 13a and lower slits 11b and 13b of the first and second diffusion rate control units 11 and 13, specifically, the path lengths L1 and L2, widths H1 and H2, and heights t11, t12, t21, and t22. Therefore, the diffusion rate control units used to calculate the average height t are the first and second diffusion rate control units 11 and 13 of the first to fourth diffusion rate control units 11, 13, 40, and 60.

[0103] In equation (1), the temperature T of the inner pump electrode 22 is an estimated value of the temperature of the inner pump electrode 22 when the limiting current Ip of the main pump unit 21 is measured and the aforementioned adjustment pump control process (the aforementioned auxiliary pump control process and main pump control process using the target value V1*) is performed. In this embodiment, the temperature T is measured as follows. First, a temperature measurement sample is prepared, in which a temperature measurement resistor element is placed in place of the inner pump electrode 22 of the sensor element 101. Next, the heater 72 of the temperature measurement sample is energized to heat the sensor element 101 to the aforementioned drive temperature Tset. When the temperature of the sensor element 101 stabilizes, the temperature calculated from the resistance value of the resistor element is measured as the temperature T. Alternatively, the heater 72 of the sensor element 101 can be energized to heat the sensor element 101 to a predetermined drive temperature Tset. When the temperature of the sensor element 101 stabilizes, the temperature T can be measured using a thermal imager or the like. In this embodiment, the temperature T of the inner pump electrode 22 is 1123K.

[0104] In equation (1), the oxygen partial pressure Poe in the measured gas is an estimated value of the oxygen partial pressure of the measured gas when the limiting current Ip of the main pump unit 21 is measured. In this embodiment, an atmosphere gas with a nitrogen base gas, an oxygen concentration of 21%, and a pressure of 1 atm is used, so the oxygen partial pressure Poe is a corresponding partial pressure. The oxygen partial pressure Pod in the first internal cavity 20 is the oxygen partial pressure corresponding to the oxygen concentration in the first internal cavity 20 adjusted by the aforementioned adjustment pump control process (the auxiliary pump control process and the main pump control process using the target value V1*). For example, the oxygen partial pressure Pod can be obtained by obtaining the voltage V0 and the oxygen partial pressure Podr of the reference gas (atmosphere) during the adjustment pump control process and applying these to the predetermined correspondence between the voltage V0, the oxygen partial pressure Podr, and the oxygen partial pressure Pod. The value obtained by subtracting the oxygen partial pressure Pod of the first internal cavity 20 from the oxygen partial pressure Poe in the measured gas (Poe - Pod) is 0.209999 atm in this embodiment.

[0105] In the gas sensor 100, the lower the average height t of the first and second diffusion rate control sections 11 and 13, the greater the diffusion resistance in the specified portion, and the higher the dependency (static pressure dependency) of the pump current Ip0 of the main pump unit 21 (the pump unit on the most upstream side of the main pump unit 21 and the auxiliary pump unit 50) on the pressure of the measured gas. In this embodiment, by configuring the sensor element 101 so that the average height t of the first and second diffusion rate control sections 11 and 13 is 0.0035 mm or greater, it is possible to suppress excessive diffusion resistance in the specified portion, thereby suppressing excessive static pressure dependency of the pump current Ip0. The inventors have confirmed this effect through experiments, analysis, and other means. As a result, it is possible to provide a sensor element 101 that suppresses excessive static pressure dependency of the pump current Ip0. In other words, by designing the path lengths L1, L2, widths H1, H2, and heights t11, t12, t21, and t22 of the upper slits 11a, 13a and lower slits 11b, 13b of the first and second diffusion rate control sections 11, 13 so that their average height t is 0.0035 mm or greater, the excessive static pressure dependence of the pump current Ip0 can be suppressed. Furthermore, when the sensor element 101 is configured so that the average height t of the first and second diffusion rate control sections 11, 13 is 0.0090 mm or greater, the excessive static pressure dependence of the pump current Ip0 can be further suppressed. Furthermore, as the average height t of the first and second diffusion rate control sections 11, 13 increases, the diffusion resistance of the predetermined portion of the gas path from the outside to the inner pump electrode 22 decreases, increasing the amount of oxygen introduced from the outside into the vicinity of the inner pump electrode 22. Consequently, the degradation rate of the inner pump electrode 22 tends to accelerate. Degradation of the inner pump electrode 22 can be caused by, for example, oxidation and sublimation of the first noble metal (e.g., Pt) contained in the inner pump electrode 22, which reduces its catalytic activity against the specific gas (NOx), thereby reducing the oxygen extraction capability of the main pump unit 21. Furthermore, evaporation of the second noble metal (e.g., Au) contained in the inner pump electrode 22 decomposes the specific gas (NOx) in the inner pump electrode 22, or adhesion of the second noble metal to the measuring electrode 44, which reduces its catalytic activity against the specific gas and reduces NOx measurement accuracy. In contrast, by configuring the sensor element 101 so that the average height t of the first and second diffusion rate control portions 11 and 13 is 0.0250 mm or less, the degradation rate of the inner pump electrode 22 can be suppressed. The inventors have confirmed this effect through experiments and analysis. As a result, a sensor element 101 can be provided that suppresses the degradation rate of the inner pump electrode 22.

[0106] In this embodiment, as described above, the element body 102 is manufactured by sintering and integrating a six-layer stack. Therefore, in many cases, the center of the upper slits 11a, 13a and the lower slits 11b, 13b bulges or recesses relative to their ends in the width direction, resulting in uneven heights t11, t12 of the upper slits 11a, 13a and heights t12, t22 of the lower slits 11b, 13b at different locations in the width direction. This makes it difficult to actually measure the heights t11, t12, t21, and t22, and thus difficult to calculate the average height t of the first and second diffusion rate control sections 11, 13 based on the heights t11, t12, t21, and t22. In response to this, the average height t of the first and second diffusion rate control sections 11, 13 can be calculated using equation (1).

[0107] Here, the correspondence between the components of this embodiment and the components of the present invention is clarified. The first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, separator 5, and second solid electrolyte layer 6 of this embodiment correspond to the solid electrolyte layers of the present invention, and the element body 102 corresponds to the element body. Furthermore, the first and second internal cavities 20 and 40 correspond to the oxygen concentration adjustment chamber, the inner pump electrode 22 and auxiliary pump electrode 51 correspond to the inner electrode, the main pump cell 21 and auxiliary pump cell 50 correspond to the adjustment pump cell, the measuring electrode 44 corresponds to the measuring electrode, and the first and second diffusion rate control units 11 and 13 correspond to the diffusion rate control unit.

[0108] The sensor element 101 included in the gas sensor 100 of the present embodiment, described in detail above, is configured such that the average height t of the first and second diffusion rate control sections 11 and 13, as determined by equation (1), is 0.0035 mm or greater. This prevents excessive diffusion resistance in a predetermined portion of the gas path from the outside to the inner pump electrode 22, thereby suppressing excessive static pressure dependence of the pump current Ip0 of the main pump unit 21. Consequently, a sensor element 101 can be provided in which excessive static pressure dependence of the pump current Ip0 is suppressed.

[0109] Furthermore, the sensor element 101 is configured such that the average height t of the first and second diffusion rate control portions 11 and 13 obtained by equation (1) is greater than 0.0090 mm. This further suppresses excessive diffusion resistance in a predetermined portion, thereby further suppressing excessive static pressure dependence of the pump current Ip0.

[0110] Furthermore, the sensor element 101 is configured such that the average height t of the first and second diffusion rate control portions 11 and 13 obtained by equation (1) is 0.0250 mm or less, thereby suppressing excessive degradation of the inner pump electrode 22 .

[0111] It should be noted that the present invention is not limited to the above-described embodiment, and can be implemented in various forms as long as it falls within the technical scope of the present invention.

[0112] For example, in the above embodiment, the inner pump electrode 22 and the auxiliary pump electrode 51 are porous cermet electrodes made of Pt and ZrO₂ containing 1% Au, while the outer pump electrode 23, the reference electrode 42, and the measuring electrode 44 are all porous cermet electrodes made of Pt and ZrO₂. However, at least one of the electrodes 22, 23, 42, 44, and 51 need not be cermet. Furthermore, at least one of the electrodes 22, 23, 42, 44, and 51 need not be porous.

[0113] In the above embodiment, the oxygen concentration adjustment chamber includes a first internal cavity 20 and a second internal cavity 40, but this is not limiting. For example, the oxygen concentration adjustment chamber may further include additional internal cavities, or one of the first internal cavity 20 and the second internal cavity 40 may be omitted. Similarly, in the above embodiment, the adjustment pump unit includes a main pump unit 21 and an auxiliary pump unit 50, but this is not limiting. For example, the adjustment pump unit may further include additional pump units, or one of the main pump unit 21 and the auxiliary pump unit 50 may be omitted. For example, if the oxygen concentration of the measured gas can be sufficiently reduced using only the main pump unit 21, the auxiliary pump unit 50 may be omitted. If the auxiliary pump unit 50 is omitted, the control unit 96 may simply perform the main pump control process as the adjustment pump control process. Furthermore, in the main pump control process, the setting of the target value V0* based on the pump current Ip1 described above may be omitted. Specifically, a predetermined target value V0* is pre-stored in the storage unit 98, and the control unit 96 performs feedback control on the voltage Vp0 of the variable power supply 24 so that the voltage V0 reaches the target value V0*, thereby controlling the main pump unit 21. In this case, for example, the voltage V0 and the oxygen partial pressure Podr of the reference gas (atmosphere) during the main pump control process are obtained, and these are applied to the predetermined correspondence between the voltage V0, the oxygen partial pressure Podr, and the oxygen partial pressure Pod, thereby obtaining the oxygen partial pressure Pod.

[0114] In the above embodiment, the element body 102 of the sensor element 101 of the gas sensor 100 includes the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, but the present invention is not limited thereto. Figure 4 The element body 201 of the sensor element 201 does not include the third internal cavity 61 . Figure 4In the element body 202 of the sensor element 201 of the illustrated modification, the gas inlet 10, first diffusion rate control section 11, buffer space 12, second diffusion rate control section 13, first internal cavity 20, third diffusion rate control section 30, and second internal cavity 40 are sequentially connected and adjacently formed between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. Furthermore, a measuring electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4 within the second internal cavity 40. The measuring electrode 44 is covered by a fourth diffusion rate control section 45. This fourth diffusion rate control section 45 is a membrane composed of a porous ceramic material such as alumina (Al2O3). Similar to the fourth diffusion rate control section 60 of the aforementioned embodiment, the fourth diffusion rate control section 45 functions to limit the amount of NOx flowing into the measuring electrode 44. Furthermore, the fourth diffusion rate control section 45 also functions as a protective film for the measuring electrode 44. The top electrode portion 51a of the auxiliary pump electrode 51 is formed directly above the measuring electrode 44. Even with the sensor element 201 having such a configuration, the NOx concentration can be detected based on, for example, the pump current Ip2, similarly to the above-described embodiment. In this case, the area around the measuring electrode 44 functions as a measuring chamber.

[0115] In the above embodiment, the outer pump electrode 23 serves as an electrode paired with the inner pump electrode 22 of the main pump cell 21 (also referred to as the outer main pump electrode), as an electrode paired with the auxiliary pump electrode 51 of the auxiliary pump cell 50 (also referred to as the outer auxiliary pump electrode), and as an electrode paired with the measurement electrode 44 of the measurement pump cell 41 (also referred to as the outer measurement electrode). However, the present invention is not limited to this. Alternatively, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided outside the element body 102 separately from the outer pump electrode 23 so as to contact the gas being measured.

[0116] In the above embodiment, the distal end portion of the element body 102 of the sensor element 101 is covered by the porous protective layer 77. However, it may be exposed without being covered by the porous protective layer 77. The inventors have confirmed through experiments and analysis that, when the distal end portion of the element body 102 is not covered by the porous protective layer 77, the static pressure dependence of the pump current Ip0 is reduced compared to when the distal end portion is covered by the porous protective layer 77. Therefore, in the case where the distal end portion of the element body 102 is not covered by the porous protective layer 77, the average height t of the first and second diffusion rate controlling sections 11 and 13, when the average height t is the same, is 0.0035 mm or greater. It should be noted that when the front end side portion of the element body 102 is not covered by the porous protective layer 77, compared with the case where it is covered by the porous protective layer 77, when the parameters other than the limiting current Ip of the main pump unit 21 in formula (1) (the path lengths L1, L2, widths H1, H2, etc. of the first and second diffusion rate control parts 11, 13) are the same, the diffusion resistance from the outside to the first internal cavity 20 becomes smaller, the limiting current Ip of the main pump unit 21 becomes larger, and the average height t of the first and second diffusion rate control parts 11, 13 obtained using formula (1) increases.

[0117] In the above embodiment, the element body 102 of the sensor element 101 includes the first diffusion rate control section 11 and the second diffusion rate control section 13. However, the present invention is not limited to this embodiment. One of the first diffusion rate control section 11 and the second diffusion rate control section 13 may be omitted, and only the other may be included. Alternatively, additional diffusion rate control sections may be included. For example, when the second diffusion rate control section 13 is omitted and only the first diffusion rate control section 11 is included, the height t of the first diffusion rate control section 11 is obtained using equation (3). When the first to nth diffusion rate control sections are included, the average height t of the first to nth diffusion rate control sections is obtained using equation (4) using the path lengths Li (i: 1 to n) and widths Hi of each of the first to nth diffusion rate control sections. Equation (4) is equivalent to the equation obtained by replacing "L / H" on the right side of equation (2) with "ΣLi / Hi".

[0118] t=L1 / H1×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10 (3)

[0119] t=ΣLi / Hi×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10 (4)

[0120] In the above embodiment, the first diffusion rate control section 11 of the element body 102 of the sensor element 101 includes an upper slit 11a and a lower slit 11b, but it may also include only one of these slits. Similarly, the second diffusion rate control section 13 includes an upper slit 13a and a lower slit 13b, but it may also include only one of these slits.

[0121] In the above embodiment, the sensor element 101 detects the NOx concentration in the measured gas. However, the sensor element 101 is not limited to detecting the concentration of a specific gas in the measured gas. For example, the specific gas concentration is not limited to NOx; the concentration of other oxides can be used as the specific gas concentration. If the specific gas is an oxide, as in the above embodiment, the specific gas itself generates oxygen when it is reduced in the third internal cavity 61. Therefore, the measurement pump cell 41 can obtain a detection value corresponding to this oxygen (e.g., pump current Ip2) and detect the specific gas concentration. Alternatively, the specific gas can be a non-oxide such as ammonia. If the specific gas is a non-oxide, the specific gas is converted into an oxide (e.g., NO in the case of ammonia). The converted gas generates oxygen when it is reduced in the third internal cavity 61. Therefore, the measurement pump cell 41 can obtain a detection value corresponding to this oxygen (e.g., pump current Ip2) and detect the specific gas concentration. For example, by having the inner pump electrode 22 of the first internal cavity 20 act as a catalyst, ammonia can be converted into NO in the first internal cavity 20.

[0122] In the above embodiment, the element body 102 of the sensor element 101 is provided as a stacked body having a plurality of solid electrolyte layers (layers 1 to 6), but the present invention is not limited thereto. The element body 102 of the sensor element 101 only needs to include at least one oxygen ion conductive solid electrolyte layer. For example, Figure 1 In the embodiment, the layers 1 to 5 other than the second solid electrolyte layer 6 may be formed of a material other than the solid electrolyte layer (e.g., a layer formed of aluminum oxide). In this case, the electrodes of the sensor element 101 only need to be provided on the second solid electrolyte layer 6. For example, Figure 1 The measuring electrode 44 only needs to be disposed on the lower surface of the second solid electrolyte layer 6. Furthermore, the reference gas introduction space 43 can be disposed in the separator 5 instead of in the first solid electrolyte layer 4, the reference gas introduction layer 48 can be disposed between the second solid electrolyte layer 6 and the separator 5 instead of between the first solid electrolyte layer 4 and the third substrate layer 3, and the reference electrode 42 can be disposed further rearward than the third internal cavity 61 and on the lower surface of the second solid electrolyte layer 6.

[0123] In the above embodiment, the control unit 96 sets the target value V0* of the voltage V0 based on the pump current Ip1 so that the pump current Ip1 reaches the target value Ip1* (feedback control), and performs feedback control on the pump voltage Vp0 so that the voltage V0 reaches the target value V0*. However, other control methods are also possible. For example, the control unit 96 may perform feedback control on the pump voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 reaches the target value Ip1*. In other words, the control unit 96 may omit the steps of obtaining the voltage V0 from the main pump control oxygen partial pressure detection sensor unit 80 and setting the target value V0*, and may directly control the pump voltage Vp0 based on the pump current Ip1 (or even control the pump current Ip0).

[0124] In the above embodiment, the form of the sensor element 101 included in the gas sensor 100 was described. Next, a method for evaluating the sensor element 101 will be described. First, the sensor element 101 to be evaluated and its corresponding temperature measurement sample are manufactured using the above-described manufacturing method, and the path lengths L1 and L2 and widths H1 and H2 of the first and second diffusion rate control sections 11 and 13 at this time are obtained. For example, the path lengths L1 and L2 and widths H1 and H2 can be used as designed, or multiple identical sensor elements 101 can be manufactured and a portion thereof cut off for measurement. Next, the limiting current Ip of the main pump unit 21, the temperature T of the inner pump electrode 22, the oxygen partial pressure Poe in the gas to be measured, and the oxygen partial pressure Pod of the first internal cavity 20 are obtained for the sensor element 101 to be evaluated and its corresponding temperature measurement sample using the above-described measurement method. The computer then inputs various data into the sensor element 101 to be evaluated, calculates the average height t of the first and second diffusion rate control sections 11 and 13 using the aforementioned equation (1), and performs evaluation using the calculated average height t of the first and second diffusion rate control sections 11 and 13. It should be noted that, as described above, equation (1) is derived from equation (2). Specifically, an evaluation is performed to determine whether the average height t of the first and second diffusion rate control sections 11 and 13 is greater than or equal to 0.0035 mm. This allows an evaluation to be performed to determine whether the excessive static pressure dependence of the pump current Ip0 can be suppressed. As a result, a sensor element 101 can be provided in which the excessive static pressure dependence of the pump current Ip0 is suppressed. It should be noted that, instead of evaluating whether the average height t of the first and second diffusion rate control sections 11 and 13 is greater than or equal to 0.0035 mm, an evaluation may be performed to determine whether the average height t of the first and second diffusion rate control sections 11 and 13 is greater than or equal to 0.0090 mm. In addition to evaluating whether the average height t of the first and second diffusion rate control sections 11 and 13 is greater than or equal to 0.0035 mm or greater than or equal to 0.0090 mm, it is also possible to evaluate whether the average height t of the first and second diffusion rate control sections 11 and 13 is less than or equal to 0.0250 mm. This allows evaluation of whether the degradation rate of the main pump unit 21, specifically the inner pump electrode 22, can be suppressed. Consequently, it is possible to provide a sensor element 101 in which the degradation rate of the inner pump electrode 22 is suppressed.

[0125] It should be noted that the computer performs the processing of calculating the average height t of the first and second diffusion rate control units 11 and 13 for the sensor element 101 to be evaluated, and the processing of performing the evaluation using the calculated average height t of the first and second diffusion rate control units 11 and 13. However, at least part of the processing may also be performed by a human.

[0126] Here, the embodiment of the evaluation method for sensor element 101 has been described. However, the embodiment may also be a program that causes one or more computers to execute the processing of the evaluation method for sensor element 101, specifically, the processing of calculating the average height t of the first and second diffusion rate control units 11 and 13 for the sensor element 101 to be evaluated, and the processing of performing the evaluation using the calculated average height t of the first and second diffusion rate control units 11 and 13. The program may be recorded on a computer-readable recording medium (e.g., a hard disk, SSD, ROM, FD, CD, DVD, etc.), may be transmitted from one computer to another via a transmission medium (communication network such as the Internet or LAN), or may be transferred in other forms.

[0127] Example

[0128] Hereinafter, a specific example of manufacturing a sensor element will be described as an embodiment. However, it should be noted that the present invention is not limited to the following embodiment.

[0129] [Experimental Examples 1 to 140]

[0130] Using the above manufacturing method, respectively make Figure 1 The sensor element 101 shown is Experimental Example 1. Furthermore, the aforementioned temperature measurement sample 1 corresponding to Experimental Example 1 was also produced. Note that in producing sensor element 101, the ceramic green sheet was formed by tape casting by mixing zirconium oxide particles to which 4 mol% of yttrium trioxide as a stabilizer was added, an organic binder, and an organic solvent. Experimental Examples 2 to 140 and corresponding temperature measurement samples 2 to 140 were produced using the same manufacturing method. In Experimental Examples 1 to 140 and temperature measurement samples 1 to 140, the path lengths L1 and L2, widths H1 and H2, and heights t11, t12, t21, and t22 of the upper slits 11a and 13a and lower slits 11b and 13b of the first and second diffusion rate control units 11 and 13 were modified.

[0131] Next, for Experimental Example 1 and temperature measurement sample 1, the limiting current Ip of the main pump unit 21, the temperature T of the inner pump electrode 22, the oxygen partial pressure Poe in the measured gas, and the oxygen partial pressure Pod in the first internal cavity 20 were obtained using the aforementioned measurement method. Furthermore, for Experimental Example 1, the average height t of the first and second diffusion rate control sections 11 and 13 was calculated using the aforementioned equation (1). The average height t of the first and second diffusion rate control sections 11 and 13 was also calculated for Experimental Examples 2 to 140 using the same method. The average height t of the first and second diffusion rate control sections 11 and 13 for Experimental Examples 1 to 140 was within the range of 0.0031 to 0.0202.

[0132] [Evaluation test]

[0133] A gas sensor 100 equipped with the sensor element 101 of Experimental Example 1 was mounted on a pipe with the tip of the sensor element 101 protruding into the pipe. Heater 72 was then energized to a temperature of 800°C, heating sensor element 101. In this state, a model gas containing nitrogen as a base gas, an oxygen concentration of 18%, a specific gas (NOx) concentration of 0 ppm, and a pressure of 101.3 kPa was prepared and flowed through the pipe as the first measurement gas. The aforementioned adjustment pump control process (auxiliary pump control using target value V1* and main pump control) was then performed to obtain the pump current Ip0 of the main pump unit 21 and set it as the first pump current Ipa. Separately, a model gas containing nitrogen as a base gas, an oxygen concentration of 18%, a specific gas (NOx) concentration of 0 ppm, and a pressure of 150.3 kPa was prepared and flowed through the pipe as the second measurement gas. Then, the adjustment pump control process is performed to obtain the pump current Ip0 of the main pump unit 21 and set it as the second pump current Ipb. After the first and second pump currents Ipa and Ipb are obtained in this way, the static pressure dependence index α, which is an index related to the static pressure dependence of the pump current Ip0, is calculated using the first and second pump currents Ipa and Ipb obtained, using formula (5). The static pressure dependence index α is also calculated for experimental examples 2 to 140 using the same method. In addition, the relationship between the average height t of the first and second diffusion rate control units 11 and 13 of experimental examples 1 to 140 and the static pressure dependence index α is shown in the figure. Figure 5 .

[0134] α=(1-Ipa / Ipb) / (1-101.3kPa / 150kPa) (5)

[0135] In the evaluation test, when the static pressure dependence index α is less than 0.10, the static pressure dependence of the pump current Ip0 is judged to be good (not too high), and when the static pressure dependence index α is greater than 0.10, the static pressure dependence of the pump current Ip0 is judged to be bad (too high). Figure 5It can be seen that the static pressure dependence index α of the sensor elements 101 in Experimental Examples 1 to 140, in which the average height t of the first and second diffusion rate control sections 11 and 13 was greater than or equal to 0.0035 mm, was less than or equal to 0.10. Furthermore, the static pressure dependence index α of the sensor elements 101 in which the average height t of the first and second diffusion rate control sections 11 and 13 was greater than or equal to 0.0090 mm, was less than or equal to 0.065. Therefore, for the sensor elements 101 in which the average height t of the first and second diffusion rate control sections 11 and 13 was greater than or equal to 0.0035 mm, it can be determined that the static pressure dependence of the pump current Ip0 was good. In particular, for the sensor elements 101 in which the average height t of the first and second diffusion rate control sections 11 and 13 was greater than or equal to 0.0090 mm, it can be determined that the static pressure dependence of the pump current Ip0 was even better.

[0136] This application claims priority based on Japanese Patent Application No. 2023-006323, filed on January 19, 2023, the entire contents of which are incorporated herein by reference.

[0137] Industrial applicability

[0138] The present invention can be used in a gas sensor that detects the concentration of a specific gas such as NOx in a measured gas such as automobile exhaust gas.

[0139] Explanation of symbols

[0140] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Separator layer, 5a, 5b Partition walls, 6 Second solid electrolyte layer, 10 Gas inlet, 11 First diffusion rate control unit, 11a, 13a Upper slits, 11b, 13b Lower slits, 12 Buffer space, 13 Second diffusion rate control unit, 20 First internal cavity, 21 Main pump unit, 22 Internal pump electrode, 22 Internal electrode, 22a Top electrode portion, 22b Bottom electrode portion, 23 External pump electrode, 24 Variable power supply, 30 Third diffusion rate control unit, 40 Second internal cavity, 41 Measurement pump unit, 42 Reference electrode, 43 Reference gas introduction space, 44 Measurement electrode, 46 Variable power supply, 48 Reference gas introduction layer, 49 Reference gas Introduction part, 49a inlet part, 50 auxiliary pump unit, 51 auxiliary pump electrode, 51a top electrode part, 51b bottom electrode part, 52 variable power supply, 60 fourth diffusion rate control part, 61 third internal cavity, 70 heater part, 71 heater connector electrode, 72 heater, 73 through hole, 74 heater insulating layer, 75 pressure release hole, 76 heater power supply, 77 porous protective layer, 80 oxygen partial pressure detection sensor unit for main pump control, 81 oxygen partial pressure detection sensor unit for auxiliary pump control, 82 oxygen partial pressure detection sensor unit for measurement pump control, 83 sensor unit, 95 control device, 96 control part, 97 CPU, 98 storage part, 100 gas sensor, 101, 201 sensor element, 102, 202 element body.

Claims

1. A sensor element for detecting the concentration of a specific gas in a gas to be measured, comprising: an element body having an oxygen ion conductive solid electrolyte layer and provided with a gas flow portion therein for introducing and flowing the gas to be measured; an adjustment pump unit having an inner electrode disposed in the oxygen concentration adjustment chamber in the measured gas flow portion and adjusting the oxygen concentration in the oxygen concentration adjustment chamber; a measuring electrode disposed in a measuring chamber downstream of the oxygen concentration adjustment chamber in the measured gas flow portion; as well as a diffusion rate control unit provided in the measured gas flow unit, which applies diffusion resistance to the measured gas from the outside and introduces the measured gas into the oxygen concentration adjustment chamber; The path length L [cm] of the diffusion rate control section, the width H [cm] of the diffusion rate control section, the limiting current Ip [A] of the adjustment pump cell, the Faraday constant F [A·sec / mol], the diffusion coefficient D [cm] of oxygen, and the diffusion coefficient D [cm] of oxygen were used. 2 / sec], gas constant R[cm 3 ·atm / mol·K], the temperature T[K] of the inner electrode, the oxygen partial pressure Poe[atm] in the measured gas, and the oxygen partial pressure Pod[atm] of the oxygen concentration adjustment chamber. The height t[mm] of the diffusion rate control unit obtained by formula (A) is 0.0035 or more, t=L / H×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10(A).

2. The sensor element according to claim 1, characterized in that The height t is greater than 0.0090.

3. The sensor element according to claim 1 or 2, characterized in that The height t is less than 0.0250.

4. The sensor element according to claim 1 or 2, characterized in that The diffusion rate control unit includes a first diffusion rate control unit to an nth diffusion rate control unit, wherein n≥2, The L / H is obtained by using the path length Li [cm] and width Hi [cm] of each of the first to nth diffusion rate control units and the sum of Li / Hi, where i is 1 to n, The height t is an average of the heights ti of the first to nth diffusion rate control sections.

5. The sensor element according to claim 1 or 2, characterized in that A plurality of the oxygen concentration adjustment chambers and the adjustment pump unit having the inner electrode are provided in series along the measured gas flow portion. The diffusion rate control unit is provided at a position upstream of the oxygen concentration adjustment chamber on the most upstream side in the measured gas flow portion. The limiting current Ip is a limiting current of the adjustment pump unit that adjusts the oxygen concentration in the oxygen concentration adjustment chamber on the most upstream side in the measured gas flow section. A gas sensor comprising the sensor element according to claim 1 or 2.

7. A method for evaluating a sensor element for detecting the concentration of a specific gas in a gas to be measured. The sensor element comprises: an element body having an oxygen ion conductive solid electrolyte layer and provided with a gas flow portion therein for introducing and flowing the gas to be measured; an adjustment pump unit having an inner electrode disposed in the oxygen concentration adjustment chamber in the measured gas flow portion and adjusting the oxygen concentration in the oxygen concentration adjustment chamber; a measuring electrode having a measuring chamber disposed downstream of the oxygen concentration adjustment chamber in the measured gas flow portion; as well as a diffusion rate control unit provided in the measured gas flow unit, which applies diffusion resistance to the measured gas from the outside and introduces the measured gas into the oxygen concentration adjustment chamber; The evaluation method performs the following steps on the sensor element to be evaluated: (a) The path length L [cm] of the diffusion rate control section, the width H [cm] of the diffusion rate control section, the limiting current Ip [A] of the adjustment pump unit, the Faraday constant F [A·sec / mol], the diffusion coefficient D [cm] of oxygen were used. 2 / sec], gas constant R[cm 3 ·atm / mol·K], the temperature T[K] of the inner electrode, the oxygen partial pressure Poe[atm] in the measured gas, and the oxygen partial pressure Pod[atm] in the oxygen concentration adjustment chamber, and using formula (B), calculate the height t[mm] of the diffusion rate control unit; and (b) using said height t for evaluation, t=L / H×Ip×1 / (4×F×D / (R×T))×1 / (Poe-Pod)×10(B). 8 . A program for causing one or more computers to execute each step of the sensor element evaluation method according to claim 6 .

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