Sensor elements and gas sensors

Abstract

To sufficiently suppress adhesion of Au to a measuring electrode.SOLUTION: A sensor element 101 comprises: an element body inside which a measurement target gas circulation unit is provided; an inside pump electrode 22 disposed in a first internal space 20 of the measurement target gas circulation unit; a measuring electrode 44 disposed in a third internal space 61 provided downstream of the first internal space 20; and an Au adsorption layer 91 including at least one of Pt and Rh and covering a part of the measuring electrode 44. The inside pump electrode 22 includes a noble metal having catalytic activity and Au. The measuring electrode 44 includes at least one of Pt and Rh. The Au adsorption layer 91 and the measuring electrode 44 have a total peak area ratio of Pt and Rh (=total peak area of a portion of the Au adsorption layer 91 where Pt or Rh is exposed / total peak area of a portion of the measuring electrode 44 where Pt or Rh is exposed at a portion not covered by the Au adsorption layer 91) measured using X-ray photoelectron spectroscopy (XPS) is 1.2 or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a sensor element and a gas sensor.

Background Art

[0002] Conventionally, a gas sensor for detecting a specific gas concentration, which is the concentration of a specific gas such as NOx in a measured gas such as exhaust gas of an automobile, is known. For example, Patent Document 1 discloses an element body provided therein with a measured gas flow portion for introducing and flowing a measured gas including an oxygen ion conductive solid electrolyte layer, an adjustment pump cell for adjusting the oxygen concentration in the oxygen concentration adjustment chamber of the measured gas flow portion, and a measurement pump cell having a measurement electrode disposed in a measurement chamber provided downstream of the oxygen concentration adjustment chamber in the measured gas flow portion, and a reference electrode. When detecting the concentration of NOx with this gas sensor, first, the oxygen concentration in the oxygen concentration adjustment chamber is adjusted by the adjustment pump cell, and the measured gas after the oxygen concentration is adjusted reaches the measurement chamber. In the measurement chamber, NOx in the measured gas is reduced around the measurement electrode. Then, the measurement pump cell is feedback-controlled so that the voltage V2 generated between the measurement electrode and the reference electrode becomes a predetermined target value to pump out oxygen around the measurement electrode. Based on the pump current Ip2 flowing at this time, the concentration of NOx in the measured gas is detected.

[0003] Furthermore, Patent Document 1 describes that the inner pump electrode, which is part of the adjustment pump cell and is located in the gas flow section to be measured, is a cermet electrode made of Pt containing 1% Au and ZrO2. The inclusion of Au in the inner pump electrode can suppress the reduction of NOx by the inner pump electrode. On the other hand, Patent Document 2 describes that when the gas sensor is used, if the Au contained in the pump electrode evaporates and adheres to the sensor electrode, the ability of the sensor electrode to decompose NOx decreases. Patent Document 2 describes that the adhesion of Au to the sensor electrode can be suppressed by forming an Au adsorption layer that adsorbs Au atoms evaporated from the pump electrode in a position where neither the sensor electrode nor the pump electrode is covered. Regarding the material of the Au adsorption layer, it is described that it is preferable to contain a metal element selected from Pt, Pd, Ni, Rh, Ir, Ta, W, and Mo as the main component, and that it is preferable to use Pt, which has high Au adsorption efficiency. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-091669 [Patent Document 2] Patent No. 6292735 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, Patent Document 2 does not examine the details of the composition of the Au adsorption layer. Our investigation revealed that even with an Au adsorption layer present, there are cases where the effect of adsorbing Au is not sufficiently obtained.

[0006] This invention was made to solve these problems, and its main objective is to sufficiently suppress the adhesion of Au to the measuring electrode. [Means for solving the problem]

[0007] To achieve the main objectives described above, the present invention employs the following means.

[0008] [1] The sensor element of the present invention is The element body has an oxygen ion conductive solid electrolyte layer and an internal gas flow section for introducing and circulating the gas to be measured, An adjustment pump electrode is provided in the oxygen concentration adjustment chamber of the gas flow section to be measured, A measuring electrode is disposed in a measuring chamber located downstream of the oxygen concentration adjustment chamber in the gas flow section to be measured, An Au adsorption layer comprising at least one of Pt and Rh, covering a portion of the measuring electrode, Equipped with, The adjustment pump electrode comprises a noble metal having catalytic activity and Au, The measuring electrode includes at least one of Pt and Rh, The Au adsorption layer and the measurement electrode have a total peak area ratio of Pt and Rh measured using X-ray photoelectron spectroscopy (XPS) of 1.2 or more (= total peak area of ​​the portion of the Au adsorption layer where Pt or Rh is exposed / total peak area of ​​the portion of the measurement electrode not covered by the Au adsorption layer where Pt or Rh is exposed). It is.

[0009] This sensor element includes an Au adsorption layer that covers a portion of the measuring electrode. Furthermore, both the measuring electrode and the Au adsorption layer contain at least one of Pt and Rh. The combined peak area ratio of Pt and Rh in the Au adsorption layer and the measuring electrode is 1.2 or higher. This combined peak area ratio of 1.2 or higher makes it easier for Au evaporated from the adjustment pump electrode to be adsorbed onto the Au adsorption layer than onto the measuring electrode, thereby effectively suppressing Au adhesion to the measuring electrode.

[0010] [2] The sensor element described above (the sensor element described in [1] above) comprises a measuring electrode lead connected to the measuring electrode, and the Au adsorption layer may be part of the measuring electrode lead. In this case, since the measuring electrode lead also serves as the Au adsorption layer, the configuration of the sensor element can be made more compact compared to the case in which the Au adsorption layer is provided separately from the measuring electrode lead.

[0011] [3] In the sensor element described above (the sensor element described in [1] or [2] above), the total peak area ratio may be 1.8 or more. When the total peak area ratio is 1.8 or more, Au is more easily adsorbed onto the Au adsorption layer than onto the measuring electrode.

[0012] [4] In the sensor element described above (the sensor element described in any of [1] to [3] above), the area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer over the measuring electrode to the area S2 of the measuring electrode, may be 1 / 20 or more. When the area ratio S1 / S2 is 1 / 20 or more, the Au adsorption layer can adsorb Au more reliably.

[0013] [5] In the sensor element described above (the sensor element described in any of [1] to [4] above), the area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer to the area S2 of the measuring electrode, may be 1 / 4 or less. When the area ratio S1 / S2 is 1 / 4 or less, there is a sufficient portion of the measuring electrode that is not covered by the Au adsorption layer, so the ability of the measuring electrode to reduce NOx is sufficient.

[0014] [6] The gas sensor of the present invention is equipped with a sensor element described in any of [1] to [5] above. Therefore, this gas sensor can obtain the same effects as the sensor element described above, for example, the effect of sufficiently suppressing the adhesion of Au to the measuring electrode. [Brief explanation of the drawing]

[0015] [Figure 1] A schematic cross-sectional diagram illustrating an example of the configuration of the gas sensor 100. [Figure 2]Cross-sectional view showing a part of the A-A cross-section of FIG. 1. [Figure 3] Front view of measurement electrode 44 and Au adsorption layer 91 as seen from the front. [Figure 4] Block diagram showing the electrical connection relationship between control device 95 and each cell and heater 72. [Figure 5] Top view showing the measurement location of X-ray photoelectron spectroscopy (XPS).

Embodiments for Carrying Out the Invention

[0016] Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing a part of the A-A cross-section of FIG. 1. FIG. 3 is a front view of measurement electrode 44 and Au adsorption layer 91 as seen from the front. FIG. 4 is a block diagram showing the electrical connection relationship between control device 95 and each cell and heater 72. This gas sensor 100 is attached to a pipe such as an exhaust gas pipe of an internal combustion engine. The gas sensor 100 detects a specific gas concentration, which is the concentration of a specific gas such as NOx or ammonia in the measured gas, with the exhaust gas of the internal combustion engine as the measured gas. In the present embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The gas sensor 100 includes a sensor element 101 having a long rectangular parallelepiped shape, cells 21, 41, 50, 80 to 83 provided in the sensor element 101, a heater section 70 provided inside the sensor element 101, variable power supplies 24, 46, 52, and a heater power supply 76, and a control device 95 that controls the entire gas sensor 100.

[0017] The sensor element 101 is an element having a laminate in which six layers, namely, a first substrate layer 1 each made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO₂), a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, are laminated in this order from the lower side in a top view. Further, the solid electrolytes forming these six layers are dense and airtight. Such a sensor element 101 is manufactured, for example, by performing predetermined processing and printing of a circuit pattern on ceramic green sheets corresponding to each layer, then laminating them, and further firing and integrating them.

[0018] On the tip side (the left end side in FIG. 1) of the sensor element 101, 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 limiting portion 11, a buffer space 12, a second diffusion rate limiting portion 13, a first internal cavity 20, a third diffusion rate limiting portion 30, a second internal cavity 40, a fourth diffusion rate limiting portion 60, and a third internal cavity 61 are adjacently formed in a communicating manner in this order.

[0019] 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 partitioned by the lower surface of the second solid electrolyte layer 6 at the upper part, the upper surface of the first solid electrolyte layer 4 at the lower part, and the side surface of the spacer layer 5 at the side part in a mode where the spacer layer 5 is hollowed out.

[0020] Each of the first diffusion rate limiting portion 11, the second diffusion rate limiting portion 13, and the third diffusion rate limiting portion 30 is provided as two horizontally long slits (the opening has a longitudinal direction in a direction perpendicular to the drawing). Further, the fourth diffusion rate limiting portion 60 is provided as one horizontally long slit (the opening has a longitudinal direction in a direction perpendicular to the drawing) 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 also referred to as the measured gas flow portion.

[0021] The sensor element 101 is equipped with a reference gas introduction section 49 that flows a reference gas to the reference electrode 42 from outside the sensor element 101 when measuring the NOx concentration. The reference gas introduction section 49 has a reference gas introduction space 43 and a reference gas introduction layer 48. The reference gas introduction space 43 is a space provided inward from the rear end surface of the sensor element 101. The reference gas introduction space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and is provided at a position where its sides are partitioned by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 opens to the rear end surface of the sensor element 101, and this opening functions as an inlet 49a of the reference gas introduction section 49. The reference gas is introduced into the reference gas introduction space 43 from this inlet 49a. The reference gas introduction section 49 introduces the reference gas introduced from the inlet 49a to the reference electrode 42 while imparting a predetermined diffusion resistance to it. In this embodiment, the reference gas is the atmosphere.

[0022] The reference gas introduction layer 48 is provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference gas introduction layer 48 is a porous material made of ceramics such as alumina. A portion of the upper surface of the reference gas introduction layer 48 is exposed within the reference gas introduction space 43. The reference gas introduction layer 48 is formed to cover the reference electrode 42. The reference gas introduction layer 48 allows the reference gas to flow from the reference gas introduction space 43 to the reference electrode 42.

[0023] The reference electrode 42 is formed in such a manner that it is 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 it. Furthermore, as will be described later, it is possible to measure the oxygen concentration (partial pressure of oxygen) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 using the reference electrode 42. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode made of Pt and ZrO2).

[0024] In the gas flow section, the gas inlet 10 is a part that opens to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas inlet 10. The first diffusion rate-limiting section 11 is a part that imparts a predetermined diffusion resistance to the gas to be measured taken in from the gas inlet 10. The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate-limiting section 11 to the second diffusion rate-limiting section 13. The second diffusion rate-limiting section 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20. When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas to be measured, which is rapidly drawn into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the gas to be measured in the external space (pulsations of exhaust pressure if the gas to be measured is automobile exhaust gas), is not directly introduced into the first internal cavity 20. Instead, the pressure fluctuations of the gas to be measured are canceled out through the first diffusion rate-limiting unit 11, the buffer space 12, and the second diffusion rate-limiting unit 13 before being introduced into the first internal cavity 20. As a result, the pressure fluctuations of the gas to be measured introduced into the first internal cavity 20 become almost negligible. The first internal cavity 20 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured introduced through the second diffusion rate-limiting unit 13. This partial pressure of oxygen is adjusted by the operation of the main pump cell 21.

[0025] The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a manner that is exposed to the external space in a region corresponding to the ceiling electrode portion 22a, and the second solid electrolyte layer 6, spacer layer 5, and first solid electrolyte layer 4 which form the current path between these electrodes.

[0026] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that partition the first internal cavity 20, and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. A side electrode portion (not shown) is formed on the side wall surface (inner surface) of the spacer layer 5 that constitutes both side walls of the first internal cavity 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and is arranged in a tunnel-shaped structure at the location where the side electrode portion is installed.

[0027] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, cermet electrodes made of Pt containing 1% Au and ZrO2). The inner pump electrode 22, which comes into contact with the gas to be measured, is formed using a material with reduced reducing ability to NOx components in the gas to be measured.

[0028] In the main pump cell 21, by applying a desired voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23, and flowing a pump current Ip0 in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal cavity 20 to the external space, or pump oxygen from the external space into the first internal cavity 20.

[0029] Furthermore, in order to detect the oxygen concentration (partial pressure of oxygen) in the atmosphere in the first internal cavity 20, an electrochemical sensor cell, i.e., a main pump control oxygen partial pressure detection sensor cell 80, is constructed using an inner pump electrode 22, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0030] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be determined by measuring the electromotive force (voltage V0) in the oxygen partial pressure detection sensor cell 80 for main pump control. Furthermore, the pump current Ip0 is controlled by feedback control of the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value. As a result, the oxygen concentration in the first internal cavity 20 can be maintained at a predetermined constant value.

[0031] The third diffusion rate-limiting section 30 is a part that applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial oxygen pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20, and guides the gas to be measured to the second internal cavity 40.

[0032] The second internal cavity 40 is provided as a space for further adjustment of the oxygen partial pressure by the auxiliary pump cell 50 for the gas to be measured, which is introduced through the third diffusion rate-limiting unit 30 after the oxygen concentration (oxygen partial pressure) has been adjusted in the first internal cavity 20. As a result, the oxygen concentration in the second internal cavity 40 can be kept constant with high precision, enabling highly accurate NOx concentration measurement in the gas sensor 100.

[0033] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell comprising an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, any suitable electrode on the outside of the sensor element 101 is sufficient), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0034] The auxiliary pump electrode 51 is disposed within the second internal cavity 40 in a tunnel-shaped structure similar to that of the inner pump electrode 22 provided within the first internal cavity 20. Specifically, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 which forms the ceiling surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 which forms the bottom surface of the second internal cavity 40. Side electrode portions 51c (see Figure 2) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both walls of the spacer layer 5 which forms the side walls of the second internal cavity 40, creating a tunnel-shaped structure. The auxiliary pump electrode 51 is also formed using a material with weakened reduction ability to the NOx component in the gas being measured, similar to the inner pump electrode 22.

[0035] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen from the atmosphere in the second internal cavity 40 to the outside space, or pump oxygen from the outside space into the second internal cavity 40.

[0036] Furthermore, in order to control the partial pressure of oxygen in the atmosphere within the second internal cavity 40, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 81 for auxiliary pump control, is constructed using an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, and a third substrate layer 3.

[0037] Furthermore, the auxiliary pump cell 50 is pumped by a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the oxygen partial pressure detection sensor cell 81 for auxiliary pump control. As a result, the oxygen partial pressure in the atmosphere inside the second internal cavity 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0038] Furthermore, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input to the oxygen partial pressure detection sensor cell 80 for main pump control as a control signal, and the aforementioned target value of its voltage V0 is controlled so that the gradient of the oxygen partial pressure in the gas to be measured, introduced from the third diffusion rate-limiting unit 30 into the second internal cavity 40, remains constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm through the action of the main pump cell 21 and the auxiliary pump cell 50.

[0039] The fourth diffusion rate-limiting section 60 is the part that applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial pressure of oxygen) is controlled by the operation of the auxiliary pump cell 50 in the second internal cavity 40, and guides the gas to be measured to the third internal cavity 61. The fourth diffusion rate-limiting section 60 plays a role in limiting the amount of NOx flowing into the third internal cavity 61.

[0040] The third internal cavity 61 is provided 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-limiting unit 60 after the oxygen concentration (partial pressure of oxygen) has been adjusted in advance in the second internal cavity 40. The NOx concentration is measured mainly in the third internal cavity 61 by the operation of the measuring pump cell 41.

[0041] The measuring pump cell 41 measures the NOx concentration in the gas to be measured within the third internal cavity 61. The measuring pump cell 41 is an electrochemical pump cell composed of a measuring electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4. The measuring electrode 44 is a porous cermet electrode made of a material that has a higher reduction capacity for NOx components in the gas to be measured than the inner pump electrode 22. The measuring electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the third internal cavity 61.

[0042] In the measuring pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 can be pumped out, and its amount can be detected as the pump current Ip2.

[0043] Furthermore, in order to detect the partial oxygen pressure around the measuring electrode 44, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 82 for controlling the measuring pump, is formed by the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump.

[0044] The gas to be measured, introduced into the second internal cavity 40, reaches the measuring electrode 44 in the third internal cavity 61 via the fourth diffusion rate-limiting section 60 under controlled conditions of oxygen partial pressure. Nitrogen oxides in the gas to be measured surrounding the measuring electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measuring pump cell 41, and at this time, the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for measuring pump control remains constant (target value). Since the amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the nitrogen oxide concentration in the gas to be measured is calculated using the pump current Ip2 in the measuring pump cell 41.

[0045] Furthermore, by combining the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 to constitute an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of NOx components in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of NOx components in the gas to be measured.

[0046] Furthermore, an electrochemical sensor cell 83 is constructed from a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, an outer pump electrode 23, and a reference electrode 42. The electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the partial pressure of oxygen in the gas being measured outside the sensor.

[0047] In a gas sensor 100 having such a configuration, the gas to be measured, whose oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measuring pump cell 41. Therefore, the NOx concentration in the gas to be measured can be determined based on the pump current Ip2 that flows as oxygen generated by the reduction of NOx is pumped out from the measuring pump cell 41, which is approximately proportional to the NOx concentration in the gas to be measured.

[0048] Furthermore, the sensor element 101 is equipped with a heater section 70 that plays a role in temperature control by heating and maintaining the sensor element 101 to enhance the oxygen ion conductivity of the solid electrolyte. The heater section 70 comprises a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, and a pressure relief hole 75.

[0049] The heater connector electrode 71 is an electrode formed in such a manner that it is in contact with the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to an external power supply, power can be supplied to the heater unit 70 from an external source.

[0050] The heater 72 is an electrical resistor formed sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater connector electrode 71 via a through-hole 73, and generates heat when power is supplied from the heater power supply 76 (see Figure 4) through the heater connector electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0051] Furthermore, the heater 72 is embedded throughout the entire area from the first internal cavity 20 to the third internal cavity 61, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated.

[0052] The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 using an insulator such as alumina. The heater insulating layer 74 is formed to provide electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0053] The pressure relief hole 75 is a portion that penetrates the third substrate layer 3 and the reference gas introduction layer 48 and communicates with the reference gas introduction space 43, and is formed for the purpose of mitigating the rise in internal pressure due to the rise in temperature within the heater insulating layer 74.

[0054] As shown in Figure 4, the control device 95 comprises the variable power supplies 24, 46, and 52 described above, the heater power supply 76 described above, and the control unit 96. The control unit 96 is a microprocessor equipped with a CPU 97 and a memory unit 98. The memory unit 98 is a non-volatile memory that can be rewritten and can store various programs and various data, for example. The memory unit 98 stores relational equations (e.g., linear or quadratic equations) or maps as the correspondence between the pump current Ip2 and the NOx concentration. Such relational equations or maps can be determined in advance through experiments. The control unit 96 receives the following inputs: voltage V0 detected by the oxygen partial pressure detection sensor cell 80 for main pump control, voltage V1 detected by the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for measuring pump control, voltage Vref detected by the sensor cell 83, pump current Ip0 detected by the main pump cell 21, pump current Ip1 detected by the auxiliary pump cell 50, and pump current Ip2 detected by the measuring pump cell 41. The control unit 96 also outputs control signals to the variable power supplies 24, 46, and 52 to control the voltages Vp0, Vp1, and Vp2 output by the variable power supplies 24, 46, and 52, thereby controlling the main pump cell 21, the measuring pump cell 41, and the auxiliary pump cell 50. The control unit 96 controls the main pump cell 21, the measuring pump cell 41, and the auxiliary pump cell 50 to adjust the oxygen concentration in the first internal cavity 20 and the second internal cavity 40, and to pump out the oxygen in the third internal cavity 61. The control unit 96 also detects the NOx concentration in the gas to be measured based on the correspondence between the pump current Ip2 and the memory unit 98. The control unit 96 controls the power supplied by the heater power supply 76 to the heater 72 by outputting a control signal to the heater power supply 76.

[0055] Here, the materials of each electrode of the sensor element 101 will be described in detail. The inner pump electrode 22, the auxiliary pump electrode 51, and the measuring electrode 44 each contain a catalytically active precious metal. Examples of catalytically active precious metals include at least one of Pt, Rh, Ir, Ru, and Pd. The outer pump electrode 23 and the reference electrode 42 also contain a catalytically active precious metal. Furthermore, the inner pump electrode 22 and the auxiliary pump electrode 51 contain Au as a precious metal having catalytic activity suppression ability to suppress the catalytic activity of the above precious metals toward a specific gas (here, NOx). As a result, the reduction ability of the inner pump electrode 22 and the auxiliary pump electrode 51 toward the NOx component in the gas to be measured is weakened. The measuring electrode 44 contains at least one of Pt and Rh as the catalytically active precious metal. It is preferable that the measuring electrode 44 contains at least Pt. The measuring electrode 44 does not need to contain any precious metals other than Pt and Rh among the catalytically active precious metals. It is preferable that the measuring electrode 44 does not contain a noble metal having the catalytic activity-inhibiting ability described above, i.e., Au. It is also preferable that the outer pump electrode 23 and the reference electrode 42 do not contain a noble metal having the catalytic activity-inhibiting ability. It is preferable that each electrode 22, 23, 42, 44, and 51 is a cermet containing a noble metal and an oxide having oxygen ion conductivity (e.g., ZrO2). It is preferable that each electrode 22, 23, 42, 44, and 51 is a porous body. In this embodiment, the inner pump electrode 22 and the auxiliary pump electrode 51 are porous cermet electrodes of Pt and ZrO2 containing 1% Au. The measuring electrode 44 does not contain Au and is a porous cermet electrode of Pt, Rh and ZrO2. The outer pump electrode 23 and the reference electrode 42 are porous cermet electrodes containing Pt and ZrO2.

[0056] In addition to the connector electrode 71 described above, several other connector electrodes (not shown) are provided on the upper and lower surfaces of the rear end of the sensor element 101. The measuring electrode 44 is electrically connected to one of these connector electrodes via the measuring electrode lead 90 shown in Figures 2 and 3. The inner pump electrode 22, outer pump electrode 23, reference electrode 42, and auxiliary pump electrode 51 are also electrically connected to the connector electrodes via leads (not shown). The sensor element 101 and the control device 95 are electrically connected via these multiple connector electrodes. The application of voltage by the variable power supplies 24, 46, and 52, as well as the detection of pump currents Ip0, Ip1, Ip2, and voltages V0, V1, V2, and Vref, are actually performed via these multiple connector electrodes and multiple leads, including the measuring electrode lead 90.

[0057] The measuring electrode lead 90 is disposed on the upper surface of the spacer layer 5. As shown in Figures 2 and 3, the measuring electrode lead 90 has a portion disposed in the third internal cavity 61 along the left-right direction, and a portion that is sandwiched between the spacer layer 5 and the first solid electrolyte layer 4, embedded in the sensor element 101, and extends to the rear. The measuring electrode lead 90 is disposed to extend, for example, to the left rear side of the sensor element 101, and is electrically connected to the connector electrode via leads (not shown) disposed on the left side and top surface of the sensor element 101.

[0058] Furthermore, as shown in Figures 1 to 3, the sensor element 101 includes an Au adsorption layer 91 that covers a portion of the measuring electrode 44. In this embodiment, the Au adsorption layer 91 is configured as part of the measuring electrode lead 90. Therefore, the entire measuring electrode lead 90 is made of the same material as the Au adsorption layer 91. A portion of the measuring electrode lead 90 is disposed within the third internal cavity 61 and, as shown in Figure 3, is bent in a stepped manner to reach the upper surface of the measuring electrode 44. Of the measuring electrode lead 90, the portion that covers the upper surface of the measuring electrode 44 functions as the Au adsorption layer 91.

[0059] The Au adsorption layer 91, like the measuring electrode 44, contains at least one of Pt and Rh. Preferably, the Au adsorption layer 91 contains at least Pt. The Au adsorption layer 91 may be a cermet having at least one of Pt and Rh and ZrO2, which is the same as the main component of the first solid electrolyte layer 4. Preferably, the Au adsorption layer 91 is a dense body. The Au adsorption layer 91 may have a porosity of, for example, 0 to 10 by volume. Preferably, the porosity of the Au adsorption layer 91 is less than 5 by volume, and more preferably 2 by volume or less.

[0060] The Au adsorption layer 91 and the measurement electrode 44 have a total peak area ratio of Pt and Rh measured using X-ray photoelectron spectroscopy (XPS) of 1.2 or higher (= total peak area of ​​the portion of the Au adsorption layer 91 where Pt or Rh is exposed / total peak area of ​​the portion of the measurement electrode 44 where Pt or Rh is exposed in the portion not covered by the Au adsorption layer 91). A larger total peak area ratio of Pt and Rh (hereinafter simply referred to as the total peak area ratio) means that the total amount of Pt and Rh present on the surface of the Au adsorption layer 91 is greater than that on the surface of the measurement electrode 44. A total peak area ratio of 1.5 or higher is preferred, and 1.8 or higher is more preferred. The total peak area ratio may be 3.0 or lower, or 2.0 or lower.

[0061] The area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer 91 to the area S2 of the measuring electrode 44, is preferably 1 / 20 or more. Area S1 is the area of ​​the measuring electrode 44 viewed from a direction perpendicular to the surface covered by the Au adsorption layer 91 (here, the top surface) (here, the area of ​​the Au adsorption layer 91 viewed from above). Area S2 is the area viewed from a direction perpendicular to the surface on which the measuring electrode 44 is arranged (here, the top surface of the first solid electrolyte layer 4) (here, the area of ​​the measuring electrode 44 viewed from above). Area S2 also includes the area of ​​the portion covered by the Au adsorption layer 91. The area ratio S1 / S2 may be 1 / 10 or more. The area ratio S1 / S2 is preferably 1 / 4 or less. Area S1 is, for example, 0.01 mm 2 More than 0.25mm 2The following may also be used: Area S2 is, for example, 0.1 mm². 2 1mm or more 2 The following is also acceptable.

[0062] Next, an example of a method for manufacturing such a gas sensor 100 is described below. First, six unfired ceramic green sheets containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component are prepared. Multiple 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-formed in the green sheet that will become the spacer layer 5 by punching or other processes. Then, a pattern printing process is performed to form various patterns on each ceramic green sheet, corresponding to the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6. Specifically, the patterns to be formed are, for example, patterns for each electrode such as the measuring electrode 44 mentioned above, lead wires such as the measuring electrode lead 90 connected to each electrode, the reference gas introduction layer 48, the connector electrode 71, and the heater part 70. Pattern printing is performed by applying a pattern-forming paste, prepared according to the characteristics required for each target of pattern formation, onto the green sheet using known screen printing technology. In this embodiment, since the Au adsorption layer 91 is part of the measuring electrode lead 90, the pattern for the Au adsorption layer 91 can be formed by forming a pattern that will become the measuring electrode lead 90 such that a part of the measuring electrode lead 90 covers the measuring electrode 44. Specifically, a pattern that will become the measuring electrode 44 is formed on a green sheet that will become the first solid electrolyte layer 4, and then a pattern that will become the measuring electrode lead 90 is formed so as to partially overlap with the pattern that will become the measuring electrode 44. After the pattern printing process, a drying process is performed using known drying means. Once the pattern printing and drying are complete, an adhesive paste is printed and dried to laminate and bond the green sheets corresponding to each layer. Then, the green sheets with the adhesive paste formed on them are positioned using sheet holes and laminated in a predetermined order, and then compressed by applying predetermined temperature and pressure conditions to form a single laminate. The laminate thus obtained contains a plurality of sensor elements 101. The laminate is cut and divided into pieces the size of the sensor elements 101. Then, the cut laminate is fired at a predetermined firing temperature to obtain the sensor elements 101.

[0063] Once the sensor element 101 is obtained in this way, a sensor assembly is manufactured by incorporating the sensor element 101 into an element encapsulant (not shown), and a protective cover and the like are attached. Then, by electrically connecting the sensor element 101 and the control device 95, a gas sensor 100 is obtained.

[0064] Furthermore, the paste used for patterning the measuring electrode 44 and the Au adsorption layer 91 (here, the measuring electrode lead 90) is a paste containing at least one of Pt and Rh particles. For example, a conductive paste prepared by mixing at least one of Pt and Rh particles with ZrO2 powder and a binder can be used for patterning the measuring electrode 44 and the Au adsorption layer 91. In addition, the total peak area ratio can be adjusted by making the total content ratio of Pt and Rh different in the patterning paste for the measuring electrode 44 and the patterning paste for the Au adsorption layer 91. For example, by making the total content ratio of Pt and Rh in the patterning paste for the Au adsorption layer 91 1.2 times or more than that of the patterning paste for the measuring electrode 44, the total peak area ratio can be made 1.2 or more. However, segregation may occur when Pt and Rh particles migrate between the measuring electrode 44 and the Au adsorption layer 91 during firing of the sensor element 101. For example, if the pattern-forming paste for the measuring electrode 44 contains Pt and Rh, and the pattern-forming paste for the Au adsorption layer 91 contains Pt but not Rh, then during firing, Rh particles may migrate from the measuring electrode 44 side to the Au adsorption layer 91 side. Similarly, Pt particles may migrate from the pattern-forming paste of the measuring electrode 44 to the pattern-forming paste of the Au adsorption layer 91, where the Pt content is higher. It is preferable to adjust the total content ratio of Pt and Rh in both pattern-forming pastes, taking into account the amount of such particle migration. The porosity of the Au adsorption layer 91 can be adjusted, for example, by adjusting the proportion of porosizing agent contained in the pattern-forming paste of the Au adsorption layer 91.

[0065] Furthermore, the area ratio S1 / S2 can be adjusted by adjusting the shape and printing position of the mask when screen printing the paste for pattern formation of the Au adsorption layer 91 (in this case, the measuring electrode lead 90) to adjust the area of ​​the overlap between the pattern of the measuring electrode 44 and the pattern of the Au adsorption layer 91. It is preferable to adjust the area of ​​the overlap during printing, taking into consideration that the area of ​​the Au adsorption layer 91 may increase during the firing of the sensor element 101. For example, if the pattern of the Au adsorption layer 91 covers (overlaps) 1 / 30 or more of the area of ​​the pattern of the measuring electrode 44 during printing, the area ratio S1 / S2 can be made 1 / 20 or more. In addition, the area ratio S1 / S2 may also be affected by the amount of Pt and Rh segregation mentioned above. For example, as described above, when Rh particles move from the measuring electrode 44 side to the Au adsorption layer 91 side during firing and segregation occurs, the larger the area ratio S1 / S2, the greater the amount of Rh segregation, and as a result, the larger the area ratio S1 / S2 tends to be, the larger the total peak area ratio tends to be. It is preferable to adjust the area ratio S1 / S2 taking this point into consideration, or to adjust the total content ratio of Pt and Rh in the pattern-forming pastes of the measuring electrode 44 and the Au adsorption layer 91.

[0066] An example of using the gas sensor 100 configured in this way is described below. The CPU 97 of the control unit 96 first controls the heater power supply 76 to supply power to the heater 72 and controls the temperature of the heater 72 so that it reaches a target temperature (e.g., 800°C). The CPU 97 acquires a value that can be converted to the temperature of the heater 72 (e.g., the resistance value or current value of the heater 72) and controls the temperature of the heater 72 by feedback control of the heater power supply 76 based on that value. When the temperature of the heater 72 reaches the target temperature (or near the target temperature), the CPU 97 starts controlling the pump cells 21, 41, and 50 as described above, and acquires the voltages V0, V1, V2, and Vref from the sensor cells 80 to 83 as described above. In this state, when the gas to be measured is introduced from the gas inlet 10, the gas to be measured passes through the first diffusion rate-limiting section 11, the buffer space 12, and the second diffusion rate-limiting section 13 and reaches the first internal space 20. Next, the oxygen concentration of the gas to be measured is adjusted by the main pump cell 21 and the auxiliary pump cell 50 in the first internal cavity 20 and the second internal cavity 40, and the adjusted gas reaches the third internal cavity 61. Then, the CPU 97 detects the NOx concentration in the gas to be measured based on the acquired pump current Ip2 and the correspondence stored in the memory unit 98.

[0067] When the control device 95 detects the NOx concentration using the sensor element 101, the Au contained in the inner pump electrode 22 and the auxiliary pump electrode 51 may evaporate due to the high temperature of the gas being measured or heat from the heater 72. Generally, when the evaporated Au adheres to the measuring electrode 44, the catalytic activity of the measuring electrode 44 toward NOx is suppressed, making it impossible to sufficiently reduce NOx around the measuring electrode 44. As a result, the actual pump current Ip2 decreases compared to the correct pump current Ip2 corresponding to the NOx concentration, and the detection accuracy of the specific gas concentration decreases. In contrast, in the gas sensor 100 of this embodiment, a part of the measuring electrode 44 is covered with an Au adsorption layer 91, and the Au adsorption layer 91 contains at least one of Pt and Rh. Furthermore, the total peak area ratio of Pt and Rh between the Au adsorption layer 91 and the measuring electrode 44 is 1.2 or more. The more Pt and Rh present on the surface of the Au adsorption layer 91, the more easily Au evaporated from the inner pump electrode 22 and auxiliary pump electrode 51 is adsorbed onto the Au adsorption layer 91 than onto the measuring electrode 44. In particular, when the total peak area ratio is 1.2 or higher, the effect of making Au more easily adsorbed onto the Au adsorption layer 91 is sufficient, and the adhesion of Au to the measuring electrode 44 is sufficiently suppressed. As a result, the decrease in the detection accuracy of NOx concentration caused by the adhesion of Au to the measuring electrode 44 when using the sensor element 101 can be sufficiently suppressed.

[0068] The larger the total peak area ratio of Pt and Rh between the Au adsorption layer 91 and the measurement electrode 44, the more easily Au is adsorbed onto the Au adsorption layer 91 than onto the measurement electrode 44. From this viewpoint, a total peak area ratio of 1.5 or higher is preferable, and 1.8 or higher is more preferable.

[0069] The larger the area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer 91 to the area S2 of the measuring electrode 44, the greater the ability of the Au adsorption layer 91 to adsorb Au. In particular, an area ratio S1 / S2 of 1 / 20 or more is preferable because the Au adsorption layer 91 can adsorb Au more reliably. From this viewpoint, the area ratio S1 / S2 may be 1 / 10 or more, or 1 / 8 or more.

[0070] If the area ratio S1 / S2 is too large, the portion of the measuring electrode 44 not covered by the Au adsorption layer 91 will decrease, which may reduce the measuring electrode 44's ability to reduce NOx. From this viewpoint, an area ratio S1 / S2 of 1 / 4 or less is preferable. The area ratio S1 / S2 may also be 1 / 8 or less, or 1 / 10 or less.

[0071] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The laminate formed by stacking the six layers of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 in this embodiment in this order corresponds to the element body of the present invention. The first internal cavity 20 and the second internal cavity 40 correspond to the oxygen concentration adjustment chamber, the inner pump electrode 22 and the auxiliary pump electrode 51 correspond to the adjustment pump electrode, the third internal cavity 61 corresponds to the measurement chamber, the measurement electrode 44 corresponds to the measurement electrode, and the Au adsorption layer 91 corresponds to the Au adsorption layer. In addition, the measurement electrode lead 90 corresponds to the measurement electrode lead, and the gas sensor 100 corresponds to the gas sensor.

[0072] As described in detail above, the gas sensor 100 of this embodiment has a total peak area ratio of 1.2 or more for Pt and Rh measured using X-ray photoelectron spectroscopy (XPS) for the Au adsorption layer 91 and the measuring electrode 44 of the sensor element 101. Therefore, Au evaporated from the inner pump electrode 22 and the auxiliary pump electrode 51 is more easily adsorbed onto the Au adsorption layer 91 than onto the measuring electrode 44, and the adhesion of Au to the measuring electrode 44 can be sufficiently suppressed. If the total peak area ratio is 1.8 or more, Au is even more easily adsorbed onto the Au adsorption layer 91 than onto the measuring electrode 44.

[0073] Furthermore, the Au adsorption layer 91 is part of the measuring electrode lead 90. In other words, the measuring electrode lead 90 also serves as the Au adsorption layer 91. This makes the sensor element 101 more compact compared to the case where the Au adsorption layer 91 is provided separately from the measuring electrode lead 90. In addition, since the measuring electrode lead 90 and the Au adsorption layer 91 can be formed integrally during the manufacturing of the sensor element 101, the manufacturing process can be simplified and manufacturing costs can be reduced.

[0074] Furthermore, if the area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer 91 to the area S2 of the measuring electrode 44, is 1 / 20 or more, the Au adsorption layer 91 can adsorb Au more reliably. If the area ratio S1 / S2 is 1 / 4 or less, there is a sufficient portion of the measuring electrode 44 that is not covered by the Au adsorption layer 91, so the ability of the measuring electrode 44 to reduce NOx is sufficient.

[0075] It goes without saying that the present invention is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

[0076] For example, in the embodiment described above, the Au adsorption layer 91 was part of the measuring electrode lead 90, but it is not limited to this. The Au adsorption layer 91 may be provided independently of the measuring electrode lead 90. In this case, the Au adsorption layer 91 does not need to be in contact with the measuring electrode lead 90. For example, a portion of the measuring electrode lead 90 may be disposed between the measuring electrode 44 and the first solid electrolyte layer 4, thereby providing electrical contact with the lower surface of the measuring electrode 44. Furthermore, the material of the Au adsorption layer 91 may be different from the material of the measuring electrode lead 90. For example, the measuring electrode lead 90 only needs to be made of a conductor and does not need to contain Pt and Rh.

[0077] In the embodiment described above, the oxygen concentration adjustment chamber had a first internal cavity 20 and a second internal cavity 40, but it is not limited to this, for example, the oxygen concentration adjustment chamber may have another internal cavity, or one of the first internal cavity 20 and the second internal cavity 40 may be omitted. Similarly, in the embodiment described above, the adjustment pump cell had a main pump cell 21 and an auxiliary pump cell 50, but it is not limited to this, for example, the adjustment pump cell may have another pump cell, or one of the main pump cell 21 and the auxiliary pump cell 50 may be omitted. For example, if the oxygen concentration of the gas to be measured can be sufficiently lowered with only the main pump cell 21, the auxiliary pump cell 50 may be omitted. If the auxiliary pump cell 50 is omitted, the control unit 96 only needs to perform the main pump control process as the adjustment pump control process. Also, 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 stored in the memory unit 98 beforehand, and the control unit 96 controls the main pump cell 21 by feedback-controlling the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes the target value V0*.

[0078] In the embodiments described above, both the inner pump electrode 22 and the auxiliary pump electrode 51 contained Au, but this is not limited to this. It is sufficient that at least one of the inner pump electrode 22 and the auxiliary pump electrode 51 contains Au. That is, if the regulating pump cell has multiple pump cells and multiple regulating pump electrodes exist, it is sufficient that at least one regulating pump electrode contains Au.

[0079] In the embodiment described above, the outer pump electrode 23 served as an electrode paired with the inner pump electrode 22 in the main pump cell 21 (also referred to as the outer main pump electrode), an electrode paired with the auxiliary pump electrode 51 in the auxiliary pump cell 50 (also referred to as the outer auxiliary pump electrode), and an electrode paired with the measuring electrode 44 in the measuring pump cell 41 (also referred to as the outer measuring electrode), but is not limited to this. One or more of the outer main pump electrode, outer auxiliary pump electrode, and outer measuring electrode may be provided separately from the outer pump electrode 23 on the outside of the element body so as to be in contact with the gas to be measured.

[0080] In the embodiment described above, the sensor element 101 is used to detect the NOx concentration in the gas to be measured, but it is not limited to this, as long as it detects the concentration of a specific gas in the gas to be measured. For example, the specific gas concentration may be other oxide concentrations, not just NOx. If the specific gas is an oxide, oxygen is generated when the specific gas itself is reduced in the third internal cavity 61, as in the embodiment described above, so the measuring pump cell 41 can detect the specific gas concentration by obtaining a detection value (e.g., pump current Ip2) corresponding to this oxygen. Alternatively, the specific gas may be a non-oxide such as ammonia. If the specific gas is a non-oxide, oxygen is generated when the converted gas is reduced in the third internal cavity 61 by converting the specific gas to an oxide (e.g., converting ammonia to NO), so the measuring pump cell 41 can detect the specific gas concentration by obtaining a detection value (e.g., pump current Ip2) corresponding to this oxygen. For example, the inner pump electrode 22 of the first internal cavity 20 functions as a catalyst, so ammonia can be converted to NO in the first internal cavity 20.

[0081] In the embodiment described above, the element body of the sensor element 101 is a laminate having a plurality of solid electrolyte layers (layers 1 to 6), but it is not limited to this. The element body of the sensor element 101 only needs to include at least one solid electrolyte layer that conducts oxygen ions. For example, in Figure 1, layers 1 to 5 other than the second solid electrolyte layer 6 may be layers made of a material other than a solid electrolyte layer (for example, a layer made of alumina). In this case, each electrode of the sensor element 101 should be arranged in the second solid electrolyte layer 6. For example, the measuring electrode 44 in Figure 1 should be arranged on the lower surface of the second solid electrolyte layer 6. Also, the reference gas introduction space 43 may be provided in the spacer layer 5 instead of the first solid electrolyte layer 4, the reference gas introduction layer 48 may be provided between the second solid electrolyte layer 6 and the spacer layer 5 instead of between the first solid electrolyte layer 4 and the third substrate layer 3, and the reference electrode 42 may be provided behind the third internal cavity 61 and on the lower surface of the second solid electrolyte layer 6. [Examples]

[0082] The following describes specific examples of how sensor elements have been fabricated. However, the present invention is not limited to the following embodiments.

[0083] [Preparation of Examples 1-3] Examples 1 to 3 were prepared by manufacturing the sensor element 101 as described above, as shown in Figures 1 to 3. Examples 1 to 3 were prepared similarly, except that the total peak area of ​​the portion of the measuring electrode 44 in which Pt or Rh is exposed was different. In preparing the sensor element 101, the green sheets for each layer 1 to 6 were formed by mixing zirconia particles with 4 mol% yttria as a stabilizer, an organic binder, and an organic solvent, and then forming them by tape molding. The paste for patterning the inner pump electrode 22 and the auxiliary pump electrode 51 was prepared by mixing a coating powder (Pt powder coated with Au), zirconia powder, and a binder. The paste for patterning the measuring electrode 44 was prepared by mixing Pt powder, Rh powder, ZrO2 powder, and a binder. The paste for patterning the measuring electrode lead 90, which includes the Au adsorption layer 91, was prepared by mixing Pt powder, ZrO2 powder, and a binder, without mixing in Rh powder. Furthermore, the Pt content in the paste for the measuring electrode lead 90 was made higher than the total Pt and Rh content in the paste for the measuring electrode 44. In addition, in Examples 1 to 3, the total Pt and Rh content in the paste for the measuring electrode 44 were made different from each other, thereby making the total peak area ratio of Pt and Rh in the Au adsorption layer 91 and the measuring electrode 44 different. In Examples 1 to 3, the area ratio S1 / S2 was set to 1 / 4.

[0084] [Examples 4, 5] Examples 4 and 5 were created by fabricating the sensor element 101 in the same manner as in Example 3, except that the area ratio S1 / S2 was different. In Example 4, the area ratio S1 / S2 was set to 1 / 10, and in Example 5, the area ratio S1 / S2 was set to 1 / 20.

[0085] [Comparative Example 1] Comparative Example 1 was prepared in the same manner as in Example 5, except that the total content ratio of Pt and Rh in the paste for the measuring electrode 44 was adjusted so that the total peak area ratio of Pt and Rh between the Au adsorption layer 91 and the measuring electrode 44 was 1.0.

[0086] [Calculation of the total peak area ratio of Pt and Rh] Multiple sensor elements 101 of Example 1 were fabricated, and one of them was cut to expose the upper surfaces of the measurement electrode 44 and the Au adsorption layer 91 to the outside. The photoelectron spectra of Pt and Rh were measured for each of the measurement electrode 44 and the Au adsorption layer 91 by X-ray photoelectron spectroscopy (XPS). The peak areas of the respective peaks of Pt and Rh in the obtained photoelectron spectra were determined, and the sum of the peak areas of Pt and Rh (total peak area of ​​Pt and Rh) in the measurement electrode 44 and the Au adsorption layer 91 was calculated. Three measurement points were used as shown in Figure 5, and the average value of the peak areas of Pt and Rh at the three points was used to calculate the total peak area. Specifically, first, a virtual line L was set that crosses the center of the front and rear of the measurement electrode 44 and the Au adsorption layer 91 in the left-right direction when viewed from above. Measurements at the measuring electrode 44 were performed at three points (black circles in Figure 5) that divided the portion of the measuring electrode 44 exposed on the virtual line L (the portion not covered by the Au adsorption layer 91) into four equal parts. Similarly, measurements at the Au adsorption layer 91 were performed at three points (white circles in Figure 5) that divided the Au adsorption layer 91 into four equal parts on the virtual line L. The total peak area of ​​Pt and Rh at the measuring electrode 44 was the peak area of ​​the detected Pt peaks at the three measurement points of the measuring electrode 44. area The total peak area of ​​Pt and Rh was calculated as the sum of the average value and the average peak area of ​​the Rh detection peak at three measurement points on the measurement electrode 44. The total peak area of ​​Pt and Rh in the Au adsorption layer 91 was calculated in the same manner. Based on these calculated total peak area values, the total peak area ratio of Pt and Rh for Example 1 (= total peak area of ​​Pt and Rh in the Au adsorption layer 91 / total peak area of ​​Pt and Rh in the portion of the measurement electrode 44 not covered by the Au adsorption layer 91) was calculated. The total peak area ratio of Pt and Rh was calculated in the same manner for Examples 2 to 5 and Comparative Example 1. MultiPak software from ULVAC-PHI, Inc. was used for data processing during peak area calculation. An asymmetric Gauss-Lorentz function was used for curve fitting analysis, and the Shirley method was used for background processing. The XPS measurement conditions are as follows.

[0087] Measurement device: PHI Quantes (Scanning X-ray Photoelectron Spectrometer) manufactured by ULVAC-PHI, Inc. X-ray source: Monochromatic Al (1486.6 eV); Detection area: 12.5 μm × 12.5 μm or larger Spectrometer: Electrostatic concentric hemispherical analyzer Extraction angle: 45° Detection spectrum (detection peaks): Pt4f, Rh3d

[0088] [Evaluation of the effect of suppressing the durability degradation of measuring electrodes] One of the multiple sensor elements 101 from Example 1 was subjected to a durability test, and the state of Au adhesion to the measurement electrode 44 and Au adsorption layer 91 after the durability test was investigated. The durability test was performed by attaching the sensor element 101 to the exhaust pipe of an experimental engine bench and exposing the sensor element 101 to exhaust gas for 2000 hours. After the durability test, the sensor element 101 was cut to expose the upper surfaces of the measurement electrode 44 and Au adsorption layer 91 to the outside, and the peak area ratio of Au (= Au peak area of ​​the Au adsorption layer 91 / Au peak area of ​​the portion of the measurement electrode 44 not covered by the Au adsorption layer 91) was calculated from the photoelectron spectrum measured by XPS, similar to the calculation of the total peak area ratio of Pt and Rh. The XPS measurement points for Au were the three points shown in Figure 5 for both the measurement electrode 44 and the Au adsorption layer 91, and the peak area of ​​Au was calculated as the average value of the peak areas of the three points. The detection spectrum (detection peak) of Au was set to the 4f peak. A larger peak area ratio of Au after the durability test indicates that more Au evaporated from the inner pump electrode 22 and auxiliary pump electrode 51 during the durability test adhered to the Au adsorption layer 91 than to the measuring electrode 44. Therefore, a larger peak area ratio of Au after the durability test indicates that the adhesion of Au to the measuring electrode is suppressed by the Au adsorption layer 91, and that the Au adsorption layer 91 has a high effect in suppressing the durability degradation of the measuring electrode 44. Accordingly, if the peak area ratio of Au is greater than 1.0, it is determined that the effect of suppressing the durability degradation of the measuring electrode 44 is high ("A"), and if the peak area ratio of Au is 1.0 or less, it is determined that there is no effect of suppressing the durability degradation of the measuring electrode 44 ("F"). The effect of suppressing durability degradation was similarly evaluated for Examples 2 to 5 and Comparative Example 1 by measuring the peak area ratio of Au after the durability test.

[0089] Table 1 summarizes the area ratio S1 / S2, the total peak area ratio of Pt and Rh, the peak area ratio of Au after the durability test, and the results of the evaluation of the durability degradation suppression effect of the measurement electrode 44 for each of Examples 1 to 5 and Comparative Example 1.

[0090] [Table 1]

[0091] As shown in Table 1, in Comparative Example 1, where the total peak area ratio of Pt and Rh was 1.0, the peak area ratio of Au after the durability test was also 1.0, indicating that no effect of suppressing the durability degradation of the measuring electrode 44 was observed. Therefore, it was found that even if the Au adsorption layer 91 contains Pt, the effect of suppressing the durability degradation of the measuring electrode 44 may not be obtained. In contrast, in Examples 1 to 5, where the total peak area ratio of Pt and Rh was 1.2 or higher, the adhesion of Au to the measuring electrode 44 was sufficiently suppressed in all cases, and the effect of suppressing the durability degradation of the measuring electrode 44 was high. Furthermore, from the results in Table 1, it is considered preferable that the area ratio S1 / S2 be 1 / 20 or higher. [Industrial applicability]

[0092] This invention can be used as a sensor element for a gas sensor that detects the concentration of specific gases such as NOx in a gas to be measured, such as automobile exhaust gas. [Explanation of Symbols]

[0093] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Spacer layer, 6 Second solid electrolyte layer, 10 Gas inlet, 11 First diffusion-controlled section, 12 Buffer space, 13 Second diffusion-controlled section, 20 First internal cavity, 21 Main pump cell, 22 Inner pump electrode, 22a Ceiling electrode section, 22b Bottom electrode section, 23 Outer pump electrode, 24 Variable power supply, 30 Third diffusion-controlled section, 40 Second internal cavity, 41 Measurement pump cell, 42 Reference electrode, 43 Reference gas introduction space, 44 Measurement electrode, 46 Variable power supply, 48 Reference gas introduction layer, 49 Reference gas introduction section, 49a Inlet section, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 51a Ceiling electrode section, 51b Bottom electrode section, 51c Side electrode section, 52 Variable power supply, 60 4th diffusion rate-limiting section, 61 3rd internal cavity, 70 heater section, 71 heater connector electrode, 72 heater, 73 through hole, 74 heater insulating layer, 75 pressure relief hole, 76 heater power supply, 80 oxygen partial pressure detection sensor cell for main pump control, 81 oxygen partial pressure detection sensor cell for auxiliary pump control, 82 oxygen partial pressure detection sensor cell for measuring pump control, 83 sensor cell, 90 measuring electrode lead, 91 Au adsorption layer, 95 control device, 96 control unit, 97 CPU, 98 memory unit, 100 gas sensor, 101 sensor element.

Claims

1. The element body has an oxygen ion conductive solid electrolyte layer and an internal gas flow section for introducing and circulating the gas to be measured, An adjustment pump electrode is provided in the oxygen concentration adjustment chamber of the gas flow section to be measured, A measuring electrode is disposed in a measuring chamber located downstream of the oxygen concentration adjustment chamber in the gas flow section to be measured, An Au adsorption layer comprising at least one of Pt and Rh, covering a portion of the measuring electrode, Equipped with, The adjustment pump electrode comprises a noble metal having catalytic activity and Au, The measuring electrode includes at least one of Pt and Rh, The Au adsorption layer and the measuring electrode have a total peak area ratio of Pt and Rh measured using X-ray photoelectron spectroscopy (XPS) of 1.2 or more (= total peak area of ​​the portion of the Au adsorption layer where Pt or Rh is exposed / total peak area of ​​the portion of the measuring electrode not covered by the Au adsorption layer where Pt or Rh is exposed). Sensor element.

2. A sensor element according to claim 1, A measuring electrode lead connected to the measuring electrode, Equipped with, The Au adsorption layer is part of the measuring electrode lead. Sensor element.

3. The above total peak area ratio is 1.8 or more. The sensor element according to claim 1 or 2.

4. The area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer to the area S2 of the measuring electrode, is 1 / 20 or more. The sensor element according to claim 1 or 2.

5. The area ratio S1 / S2, which is the ratio of the area S1 covered by the Au adsorption layer to the area S2 of the measuring electrode, is 1 / 4 or less. The sensor element according to claim 1 or 2.

6. A gas sensor comprising the sensor element according to claim 1 or 2.

Images