Sensor element

Abstract

To provide a sensor element having high water resistance while keeping adhesion strength between an element body and a protection layer.SOLUTION: A sensor element 101 includes an element body 102 including a base body part 103 with a long plate shape, and a measurement gas flow space 15 formed on a side at one end of the base body part 103 in a longitudinal direction, and a porous protection layer 90 formed from the one end and covering a predetermined length of a surface of the element body 102 in the longitudinal direction. The element main body 102 includes a space external electrode 23 disposed on one main surface. The protection layer 90 includes an inner layer 91 covering the element body 102, and an outer layer 92 existing on the outside with respect to the inner layer 91 and includes an electrode presence region where the space external electrode 23 is present and a rear region subsequent thereto in the longitudinal direction. The porosity of the rear region of the inner layer 91 is lower than that of the electrode presence region of the inner layer 91. The porosity of the outer layer 92 is lower than that of the rear region of the inner layer 91.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a sensor element for detecting a target gas in a measurement gas. [Background technology]

[0002] Gas sensors are used to detect and measure the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in gases to be measured, such as automobile exhaust gases. Known examples of such gas sensors include gas sensors equipped with a sensor element that uses an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2).

[0003] In such gas sensors, it is known to form a porous protective layer on the surface of the sensor element to prevent cracks from occurring in the internal structure of the sensor element due to thermal shock caused by moisture adhering to the sensor element. That is, a sensor element is known that includes an element body and a porous protective layer covering the element body. For example, Japanese Patent Laid-Open Publication No. 2014-098590 and International Publication No. WO 2020 / 203029 disclose a multi-layer protective layer that includes an inner layer and an outer layer with a lower porosity than the inner layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-098590 [Patent Document 2] International Publication No. WO2020 / 203029 Summary of the Invention [Problem to be solved by the invention]

[0005] When a gas sensor measures a target gas, the sensor element reaches a high temperature (for example, about 800°C). If moisture adheres to such a high-temperature sensor element, the thermal shock can cause cracks in the internal structure of the sensor element.

[0006] In a protective layer including an inner layer and an outer layer with a lower porosity than the inner layer (for example, JP 2014-098590 A and WO 2020 / 203029 A), increasing the porosity of the inner layer improves the heat insulating performance of the protective layer. As a result, cracks in the internal structure of the sensor element can be further suppressed when the sensor element is splashed with water (water exposure). In other words, the water exposure resistance of the sensor element can be improved.

[0007] However, there is a concern that the higher the porosity of the inner layer of the protective layer, the lower the adhesive strength between the element body and the inner layer.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sensor element that has high water resistance while maintaining the adhesive strength between the element body and the protective layer. [Means for solving the problem]

[0009] After extensive research, the inventors have discovered that in a porous protective layer covering at least a portion of the longitudinal direction of the surface of the element body, by making the porosity in the rear region of the inner layer of the protective layer lower than the porosity in the region where the electrode is present (the region of the inner layer that is approximately central in the longitudinal direction of the element body), it is possible to maintain high water resistance while maintaining the adhesion strength between the element body and the protective layer.

[0010] The present invention includes the following inventions.

[0011] (1) An element body including a long, plate-shaped substrate including an oxygen ion conductive solid electrolyte layer and a measurement gas flow space formed on one end of the substrate in the longitudinal direction; a porous protective layer formed from the one longitudinal end of the base portion and covering a predetermined length of the surface of the element body in the longitudinal direction; A sensor element comprising: the element body includes an intra-space electrode disposed in the measurement gas flow space, and an outer-space electrode having a predetermined length in the longitudinal direction of the base portion and disposed on one of two main surfaces of the element body in correspondence with the intra-space electrode, the protective layer includes an inner layer covering the element body and an outer layer located outside the inner layer, and has, in the longitudinal direction of the base portion, an electrode presence region in which the outer-space electrodes are present, and a rear region following the electrode presence region; the porosity of the inner layer in the rear region is lower than the porosity of the inner layer in the electrode presence region; A sensor element for detecting a measurement target gas in a measurement gas, wherein the porosity of the outer layer is lower than the porosity of the rear region of the inner layer.

[0012] (2) The sensor element according to (1), wherein the rear region of the protective layer extends in the longitudinal direction of the base portion to a position farther from the one end of the base portion in the longitudinal direction than the measurement gas flow space.

[0013] (3) The sensor element according to (2), wherein the porosity of the rear region of the protective layer in a region farther from the one end of the base portion in the longitudinal direction than the measurement gas flow space in the longitudinal direction of the base portion is lower than the porosity of the electrode presence region of the inner layer.

[0014] That is, in the above (3), the porosity in the rearmost region of the rear region, which is farther from the one longitudinal end of the base portion than the measurement gas flow space, is lower than the porosity in the electrode presence region of the inner layer.

[0015] (4) The sensor element according to any one of (1) to (3) above, wherein the porosity in the rear region of the inner layer is 30% by volume or more and 50% by volume or less.

[0016] (5) The sensor element according to any one of (1) to (4) above, wherein the porosity in the electrode presence region of the inner layer is 40% by volume or more and 80% by volume or less, provided that the porosity is higher than the porosity in the rear region of the inner layer.

[0017] (6) The sensor element according to any one of (1) to (5) above, wherein the porosity of the rear region of the inner layer is lower by 5% by volume or more than the porosity of the electrode-existing region of the inner layer.

[0018] (7) The rear region of the protective layer extends in the longitudinal direction of the base portion to a position far from the one end of the measurement target gas flow space in the longitudinal direction of the base portion, The sensor element according to any one of (1) to (6) above, wherein the porosity of the rear region of the inner layer is lower by 5% or more than the porosity of the electrode-existing region of the inner layer.

[0019] (8) The sensor element according to any one of (1) to (7), wherein the porosity of the inner layer decreases stepwise or continuously in the longitudinal direction of the base portion from the one end of the base portion in the longitudinal direction. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a sensor element that has high water resistance while maintaining the adhesive strength between the element body and the protective layer. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a perspective view showing an example of a schematic configuration of a sensor element 101. [Figure 2] 2 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including a sensor element 101, the cross-sectional view including a schematic cross-sectional view of the sensor element 101 taken along line II-II in FIG. [Figure 3]3 is a cross-sectional view showing the structure of the porous protective layer 90, taken along the same cross section as in FIG. 2. In FIG. 3, the structure inside the element body 102, other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode, is omitted from the illustration. [Figure 4] 4 is the same cross-sectional view as in FIG. 3, showing the structure of the porous protective layer 90 (inner layer 91 and outer layer 92). In FIG. 4, similar to FIG. 3, the structure inside the element body 102 other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode is omitted. [Figure 5] 5 is the same cross-sectional view as in FIG. 3, showing the structure of the porous protective layer 90 (inner layer 91 and outer layer 92). In FIG. 5, similar to FIG. 3, the structure inside the element body 102 other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode is omitted. [Figure 6] 6 is a cross-sectional view, taken along the same line as in FIGS. 3 to 5, illustrating the structure of a porous protective layer 290 (an inner layer 291 and an outer layer 92) of another example of a sensor element 201 having a different porous protective layer structure. Similar to FIG. 3, FIG. 6 does not illustrate the structure of the element body 102 other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode. [Figure 7] 10 is a flowchart illustrating an example of a method for manufacturing a sensor element. DETAILED DESCRIPTION OF THE INVENTION

[0022] The sensor element of the present invention comprises: an element body including a long, plate-like substrate including an oxygen ion conductive solid electrolyte layer and a measurement gas flow space formed on one end of the substrate in the longitudinal direction; a porous protective layer formed from the one longitudinal end of the base portion and covering a predetermined length of the surface of the element body in the longitudinal direction; Includes:

[0023] The element body includes an intra-space electrode disposed in the measurement gas flow space, and an outer-space electrode having a predetermined length in the longitudinal direction of the base portion disposed on one of the two main surfaces of the element body, corresponding to the intra-space electrode.

[0024] the protective layer includes an inner layer covering the element body and an outer layer located outside the inner layer, and has, in the longitudinal direction of the base portion, an electrode presence region in which the outer-space electrodes are present, and a rear region following the electrode presence region; the porosity of the inner layer in the rear region is lower than the porosity of the inner layer in the electrode presence region; The porosity of the outer layer is lower than the porosity of the inner layer in the rear region.

[0025] An embodiment of a gas sensor including a sensor element of the present invention will be described in detail below.

[0026] [Outline of gas sensor configuration] Embodiments of a gas sensor of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing an example of the general configuration of a sensor element 101. FIG. 2 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including the sensor element 101. In FIG. 2, the cross-sectional view of the sensor element 101 is a schematic cross-sectional view taken along line II-II in FIG. 1. In the following, with reference to FIG. 2, the upper side of FIG. 2 is referred to as the top, the lower side as the bottom, the left side of FIG. 2 as the leading end, and the right side as the rear end. Furthermore, with reference to FIG. 2, the front side perpendicular to the plane of the paper is referred to as the right, and the back side as the left.

[0027] In FIG. 2, a gas sensor 100 is an example of a limiting current type NOx sensor that detects NOx in a measurement gas by a sensor element 101 and measures its concentration.

[0028] The sensor element 101 includes a porous protective layer 90, which will be described in detail later. The porous protective layer 90 corresponds to the protective layer of the present invention. The portion of the sensor element 101 excluding the porous protective layer 90 will be referred to as an element body 102 below. The element body 102 is in the shape of a long plate. As shown in FIG. 1 , the element body 102 has six surfaces: two main surfaces (an upper surface 102a and a lower surface 102b), two side surfaces along the longitudinal direction (a left surface 102c and a right surface 102d), and two end surfaces along the longitudinal direction (a front end surface 102e and a rear end surface 102f).

[0029] The element body 102 of the sensor element 101 includes an intra-space electrode disposed in a measurement gas flow space 15 (described later) and an outer-space electrode disposed on one of the two main surfaces of the element body 102, corresponding to the intra-space electrode, and having a predetermined length in the longitudinal direction of the base portion 103. The measurement gas flow space 15 also has a predetermined length in the longitudinal direction of the base portion 103. "Corresponding to the intra-space electrode" means that the outer-space electrode is disposed so as to be in contact with the intra-space electrode via a solid electrolyte layer.

[0030] In the sensor element 101 of this embodiment, the electrodes inside the cavity are an inner main pump electrode 22, an auxiliary pump electrode 51, and a measurement electrode 44. The electrode outside the cavity is an outer pump electrode 23 disposed on the upper surface 102a.

[0031] The sensor element 101 is a long, plate-like element including a base portion 103 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-like shape is also referred to as a long plate shape or a strip shape. The base portion 103 has a structure in which six layers are stacked in this order from bottom to top as viewed in the drawing: a first substrate layer 1, 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, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO). The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness or may have different thicknesses. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 103 includes the adhesive layers. While FIG. 2 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number and layer structure may be used.

[0032] A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip end) 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. The measurement gas flow space 15, i.e., the measurement gas flow portion, is formed in the longitudinal direction from the gas inlet 10 with a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal space 20, a third diffusion-controlling section 30, a second internal space 40, a fourth diffusion-controlling section 60, and a third internal space 61 adjacently and communicating with each other in this order.

[0033] The gas inlet 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0034] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 are each provided as two horizontally elongated slits (the openings have their longitudinal direction perpendicular to the plane of the drawing in FIG. 2). The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 may have any shape that provides the desired diffusion resistance, and the shape is not limited to the slits.

[0035] The fourth diffusion rate-controlling portion 60 is provided as a single horizontally elongated slit (the opening has its longitudinal direction perpendicular to the plane of the drawing in FIG. 2 ) between the spacer layer 5 and the second solid electrolyte layer 6. The fourth diffusion rate-controlling portion 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slit.

[0036] Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measurement gas flow space 15, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position defined at its side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0037] The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0038] The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, the air introduction layer 48, which is connected to the reference gas introduction space 43, is provided around the reference electrode 42. That is, the reference electrode 42 is disposed so as to come into contact with the reference gas via the porous air introduction layer 48 and the reference gas introduction space 43. As will be described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20, the second internal space 40, and the third internal space 61. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO).

[0039] In the measurement gas flow space 15, the gas inlet 10 is open to the external space, and the measurement gas is taken into the sensor element 101 from the external space through the gas inlet 10.

[0040] In this embodiment, the measurement gas is introduced into the measurement gas flow space 15 through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this. For example, the measurement gas flow space 15 may not have a recess for the gas inlet 10. In this case, the first diffusion rate-controlling part 11 essentially serves as the gas inlet.

[0041] Furthermore, for example, the measurement gas flow space 15 may have an opening in a side surface along the longitudinal direction of the base part 103, the opening communicating with the buffer space 12 or a position in the first internal space 20 close to the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 103 through the opening.

[0042] Furthermore, for example, the measurement gas flow space 15 may be configured so that the measurement gas is introduced through a porous body.

[0043] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

[0044] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0045] The second diffusion rate-controlling portion 13 is a portion that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0046] As a result, it is sufficient that the amount of the measurement gas introduced into the first internal space 20 is within a predetermined range. That is, it is sufficient that a predetermined diffusion resistance is imparted to the entire area from the tip of the sensor element 101 to the second diffusion-controlling section 13. For example, it is also possible that the first diffusion-controlling section 11 directly communicates with the first internal space 20, that is, the buffer space 12 and the second diffusion-controlling section 13 do not exist.

[0047] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates.

[0048] When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into the first internal space 20, but is introduced into the first internal space 20 after the pressure fluctuations of the measurement gas are canceled out through the first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13. As a result, the pressure fluctuations of the measurement gas introduced into the first internal space become almost negligible.

[0049] The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling part 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0050] The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 disposed in the measurement gas flow space 15 and an outer pump electrode 23 disposed outside the space so as to be in contact with the inner main pump electrode 22 via a solid electrolyte (second solid electrolyte layer 6 in FIG. 2). The outer pump electrode 23 is disposed on the upper surface 102a of the two main surfaces of the element body 102.

[0051] That is, the main pump cell 21 is an electrochemical pump cell including an inner main 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 space 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0052] The inner main pump electrode 22 is disposed facing the first internal space 20. That is, the inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 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 space 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. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are disposed in a tunnel-like structure at the locations where the side electrode portions are disposed.

[0053] The inner main pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, a cermet electrode of Pt and ZrO containing 1% Au). The inner main pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0054] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24 and flowing a pump current Ip0 in a positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal space 20 out to the external space or pump oxygen from the external space into the first internal space 20.

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

[0056] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback-controlling the voltage Vp0 of the variable power supply 24 so that the electromotive force V0 is constant. This allows the oxygen concentration in the first internal space 20 to be maintained at a predetermined constant value.

[0057] The third diffusion control section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0058] The second internal space 40 is provided as a space for adjusting with higher precision the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling section 30. The oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. The second internal space 40 and the auxiliary pump cell 50 may be omitted. From the viewpoint of the accuracy of adjusting the oxygen partial pressure, it is more preferable to have the second internal space 40 and the auxiliary pump cell 50.

[0059] In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the measurement gas introduced through the third diffusion-controlling part is further subjected to adjustment of the oxygen partial pressure by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high precision, thereby enabling the gas sensor 100 to measure the NOx concentration with high precision.

[0060] The auxiliary pump cell 50 is an electrochemical pump cell including an auxiliary pump electrode 51 as an inner-space electrode disposed in the measurement gas flow space 15 at a position farther from the tip of the base portion 103 than the inner main pump electrode 22, and an outer pump electrode 23 as an outer-space electrode disposed so as to be in contact with the auxiliary pump electrode 51 via a solid electrolyte (second solid electrolyte layer 6 in FIG. 2 ).

[0061] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell that includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode located at a position other than within the measurement gas flow space 15, for example, outside the sensor element 101, will suffice), and the second solid electrolyte layer 6.

[0062] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to that of the inner main pump electrode 22 provided in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, forming a tunnel-shaped structure.

[0063] Like the inner main pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0064] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0065] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is configured by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0066] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere within the second internal space 40 to be controlled to a low level that does not substantially affect the measurement of NOx.

[0067] In addition, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and its electromotive force V0 is controlled to always maintain a constant gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0068] The fourth diffusion rate-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been further controlled to a lower level by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61.

[0069] The third internal space 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the fourth diffusion-controlling section 60. The NOx concentration is measured by the operation of the measurement pump cell 41.

[0070] The measurement pump cell 41 measures the NOx concentration in the measurement gas in the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including: a measurement electrode 44 as an intra-space electrode disposed in the measurement gas flow space 15 at a position farther from the tip of the base portion 103 than the auxiliary pump electrode 51; and an outer pump electrode 23 as an outer-space electrode disposed so as to be in contact with the measurement electrode 44 via a solid electrolyte (in FIG. 2 , the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4).

[0071] That is, the measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode located at a position other than within the measurement gas flow space 15, for example, outside the sensor element 101), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0072] The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61. The measurement electrode 44 contains, as a metal component, a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd).

[0073] In the measuring pump cell 41, oxygen produced by decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 is pumped out, and the amount of oxygen produced can be detected as a pump current Ip2.

[0074] In addition, in order to detect the oxygen partial pressure around the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump. The variable power supply 46 is controlled based on the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0075] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion-controlling part 60 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measurement pump cell 41, and the voltage Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant. Because the amount of oxygen generated around the measurement electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measurement pump cell 41 can be used to calculate the nitrogen oxide concentration in the measurement gas.

[0076] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 to form 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 reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0077] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0078] In the gas sensor 100 having such a configuration, the measurement gas, in which the 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 measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41, which is approximately proportional to the NOx concentration in the measurement gas.

[0079] Furthermore, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 is provided with a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0080] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply, which is an external power supply, it is possible to supply power to the heater section 70 from outside.

[0081] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the rear end side of the sensor element 101 in the longitudinal direction, and a through hole 73. The heater 72 generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0082] The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated. The temperature needs to be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary to adjust these entire areas to the same temperature, and the sensor element 101 may have a temperature distribution.

[0083] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 103, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 103. That is, the heater 72 may be capable of heating the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity that enables the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 103 as in this embodiment. Alternatively, the heater portion 70 may be formed as a heater substrate separate from the base portion 103 and disposed adjacent to the base portion 103.

[0084] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72 and heater leads 76. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and heater leads 76, and between the third substrate layer 3 and the heater 72 and heater leads 76.

[0085] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 and the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies an increase in temperature within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0086] (protective layer) The sensor element 101 includes the above-described element body 102 and a porous protective layer 90 formed from one longitudinal end (tip) of the element body 102 (base portion 103) and covering a predetermined length L of the surface of the element body 102 in the longitudinal direction. Here, the one longitudinal end of the element body 102 is the end on the side where the measurement gas flow space 15 is formed, i.e., the tip of the element body 102. The element body 102 has a long plate shape, and an upper surface 102a and a lower surface 102b of the element body 102 are main surfaces. The left surface 102c and the right surface 102d are sometimes referred to as side surfaces, and the front surface 102e and the rear surface 102f are sometimes referred to as end surfaces.

[0087] The porous protective layer 90 includes an inner layer 91 that covers the element body 102, and an outer layer 92 that is located outside the inner layer 91, and has, in the longitudinal direction of the element body 102 (base portion 103), an electrode presence region in which the outer space electrode (in this embodiment, the outer pump electrode 23) is present, and a rear region that continues from the electrode presence region.

[0088] FIG. 3 is a cross-sectional view of the porous protective layer 90 taken along the same cross section as FIG. 2, showing the configuration of the porous protective layer 90. In FIG. 3, components other than the measurement gas flow space 15, the cavity electrode, and the cavity outer electrode within the element body 102 are not shown. As shown in FIG. 3, a region of the porous protective layer 90 in the longitudinal direction of the element body 102 (base portion 103) where the outer pump electrode 23 is present is referred to as an electrode presence region 90b. A region of the porous protective layer 90 extending from the front end of the element body 102 (base portion 103) to the front end of the electrode presence region 90b is referred to as a front region 90a, and a region of the porous protective layer 90 continuing from the electrode presence region 90b, i.e., a region from the rear end of the electrode presence region 90b to the rear end of the porous protective layer 90, is referred to as a rear region 90c. That is, the rear end of the front region 90a and the front end of the electrode presence region 90b are in contact, and the rear end of the electrode presence region 90b and the front end of the rear region 90c are in contact. In the porous protective layer 90, the porosity of the rear region 91c of the inner layer 91 is lower than the porosity of the electrode presence region 91b of the inner layer 91. Furthermore, the porosity of the outer layer 92 is lower than the porosity of the rear region 91c of the inner layer 91. The porosity of the outer layer 92 is usually lower than the porosity at any position in the inner layer 91. Porosity will be described in detail later.

[0089] In this embodiment, the porous protective layer 90 covers a predetermined range of the element body 102 in the longitudinal direction from the tip of the element body 102 (the portion indicated by the dashed line in FIG. 1 ). More specifically, the porous protective layer 90 covers the entire upper surface 102a of the element body 102, a region having a predetermined length L in the longitudinal direction from the tip of the element body 102. The porous protective layer 90 covers the entire lower surface 102b of the element body 102, a region having a length L in the longitudinal direction from the tip of the element body 102. The porous protective layer 90 covers the entire left surface 102c of the element body 102, a region having a length L in the longitudinal direction from the tip of the element body 102. The porous protective layer 90 covers the entire right surface 102d of the element body 102, a region having a length L in the longitudinal direction from the tip of the element body 102. The porous protective layer 90 covers the entire tip surface 102e of the element body 102.

[0090] 2, the porous protective layer 90 also covers the gas inlet 10. However, because the porous protective layer 90 is porous, the gas to be measured can flow through the porous protective layer 90 and reach the gas inlet 10. Therefore, the gas to be measured can be detected and measured without any problems.

[0091] The porous protective layer 90 serves to prevent cracks from occurring in the internal structure of the element body 102, for example, when water splashes on the high-temperature sensor element 101 during operation of the gas sensor. Water that reaches the sensor element 101 does not adhere directly to the surface of the element body 102, but adheres to the porous protective layer 90. The surface of the porous protective layer 90 is rapidly cooled by the adhering water, but the thermal shock applied to the element body 102 is reduced due to the insulating effect of the porous protective layer 90. As a result, cracks can be prevented from occurring in the internal structure of the element body 102. In other words, the water resistance of the sensor element 101 is improved.

[0092] The porous protective layer 90 preferably covers the outer pump electrode 23. The porous protective layer 90 also serves to prevent oil components and the like contained in the gas under measurement from adhering to the outer pump electrode 23, thereby suppressing deterioration of the outer pump electrode 23.

[0093] The porous protective layer 90 is made of a porous body. Examples of materials for the porous protective layer 90 include alumina, zirconia, spinel, cordierite, mullite, titania, and magnesia. The porous protective layer 90 may be made of one or more of these materials. The materials for the inner layer 91 and the outer layer 92 may be the same or different from each other. In this embodiment, the porous protective layer 90 (the inner layer 91 and the outer layer 92) is made of a porous alumina body.

[0094] The inner layer 91 is formed to cover a predetermined range of the element body 102 in the longitudinal direction from the tip of the element body 102. More specifically, the inner layer 91 covers the entire upper surface 102a of the element body 102, a region having a predetermined length LA in the longitudinal direction from the tip of the element body 102. The inner layer 91 covers the entire lower surface 102b of the element body 102, a region having a length LA in the longitudinal direction from the tip of the element body 102. The inner layer 91 covers the entire left surface 102c of the element body 102, a region having a length LA in the longitudinal direction from the tip of the element body 102. The inner layer 91 covers the entire right surface 102d of the element body 102, a region having a length LA in the longitudinal direction from the tip of the element body 102. The inner layer 91 covers the entire tip surface 102e of the element body 102. In this embodiment, the longitudinal length LA of the inner layer 91 is equal to the longitudinal length L of the porous protective layer 90 (LA=L). Note that, hereinafter, the upper surface 102a will also be referred to as the pump surface 102a. Furthermore, of the two main surfaces of the element body 102, the main surface (lower surface 102b) opposite to the pump surface 102a will also be referred to as the heater surface 102b.

[0095] The outer layer 92 is located outside the inner layer 91. In the present embodiment, the outer layer 92 is formed so as to generally cover the surface of the inner layer 91. That is, the outer layer 92 is formed so as to cover a predetermined range in the longitudinal direction from the tip of the inner layer 91 of the element body 102. In the present embodiment, the outer layer 92 covers the entire region of the inner layer 91 from the tip of the element body 102 to the position of the longitudinal length LB from the tip of the element body 102. In the present embodiment, the longitudinal length LB of the outer layer 92 is shorter than the longitudinal length LA of the inner layer 91 (LB < LA). That is, the rear end portion of the inner layer 91 is exposed without being covered by the outer layer 92. However, the longitudinal length LB of the outer layer 92 is not limited to this. The longitudinal length LB of the outer layer 92 may be the same as the longitudinal length LA of the inner layer 91 (LB = LA). Also, it may be longer than the longitudinal length LA of the inner layer 91 (LB > LA). That is, the inner layer 91 may be completely covered by the outer layer 92 (in this case, LA < LB = L). In the present embodiment, on each of the two main surfaces (the pump surface 102a and the heater surface 102b) and the two side surfaces (the left surface 102c and the right surface 102d), the longitudinal length LB of the outer layer 92 is the same, but the longitudinal length LB of the outer layer 92 may be different from each other on the two main surfaces and the two side surfaces.

[0096] Thus, the porous protective layer 90 includes an inner layer 91 that covers the element body 102 and an outer layer 92 that is located outside the inner layer 91. The porosity of the outer layer 92 is lower than the porosity at any position of the inner layer 91. In the porous protective layer 90, the inner layer 91 mainly serves to suppress heat conduction from the surface of the sensor element 101 (the surface of the porous protective layer 90) to the element body 102. That is, the inner layer 91 has a function of reducing the thermal shock applied to the element body 102 due to the heat insulating effect of the inner layer 91. Also, the outer layer 92 having a lower porosity than the inner layer 91 mainly has a function of maintaining the structural strength of the entire porous protective layer 90. Further, the outer layer 92 also has a function of preventing water droplets from entering the inside of the porous protective layer 90 (the inside of the inner layer 91). Thereby, evaporation of water droplets inside the inner layer 91 can be prevented, and the thermal shock applied to the element body 102 can be reduced.

[0097] As described above, the porous protective layer 90 has a predetermined length L in the longitudinal direction of the element body 102 (base portion 103), and includes an electrode presence region 90b where the outer pump electrode 23 is present, and a rear region 90c continuing from the electrode presence region 90b. The inner layer 91 of the porous protective layer 90 has a predetermined length LA in the longitudinal direction of the element body 102 (base portion 103), and extends to an electrode presence region 91b where the outer pump electrode 23 is present, and a rear region 91c continuing from the electrode presence region 91b. In this embodiment, L=LA.

[0098] The porous protective layer 90 (particularly the inner layer 91) may cover a portion of the element body 102 that becomes hot when the gas sensor is in operation. The longitudinal length L of the entire porous protective layer 90 and the longitudinal length LA of the inner layer 91 may be appropriately determined within a range longer than the longitudinal length from the front end surface of the element body 102 to the rear end of the electrode presence regions 90b, 91b (the rear electrode end of the outer pump electrode 23) and shorter than the entire longitudinal length of the element body 102, based on the range of the element body 102 exposed to the measurement gas in the gas sensor 100, the position of the outer pump electrode 23, the position of the measurement gas flow space 15, the temperature distribution of the element body 102, and the like.

[0099] The inner layer 91 covers the element body 102 from the tip of the element body 102 to a region rearward of the electrode presence region 91b where the outer pump electrode 23 is present. Preferably, the inner layer 91 covers at least the region from the tip of the element body 102 to the rear end of the measurement gas flow space 15. That is, the rear region 91c of the inner layer 91 (the rear region 90c of the porous protective layer 90) extends in the longitudinal direction of the base portion 103 to a position farther from the one longitudinal end (tip) of the base portion 103 than the measurement gas flow space 15. When the gas sensor is operated, the position where the measurement gas flow space 15 is present is usually heated to a temperature high enough to develop oxygen ion conductivity of the solid electrolyte. Covering such an area with the inner layer 91 effectively reduces thermal shock applied to the heated portion of the element body 102.

[0100] Further, for example, the inner layer 91 may cover substantially the entire portion of the element body 102 that is exposed to the gas to be measured. For example, the inner layer 91 may be formed to cover substantially from the tip of the element body 102 to the longitudinal position where the reference electrode 42 is formed. Alternatively, for example, it may be formed to cover substantially from the tip of the element body 102 to the longitudinal position of the side surface on the tip side of the reference gas introduction space 43. Alternatively, it may cover even further rearward. In the present embodiment, the longitudinal length LA of the inner layer 91 is the same on each of the two main surfaces (the pump surface 102a and the heater surface 102b) and the two side surfaces (the left surface 102c and the right surface 102d), but the longitudinal length LA of the inner layer 91 may be different from each other on the two main surfaces and the two side surfaces.

[0101] The longitudinal length LA of the inner layer 91 from the tip of the element body 102 varies depending on the configuration of the element body 102, but may be, for example, 7 mm or more, 9 mm or more, or 10 mm or more. Also, the length LA may be, for example, 17 mm or less, or 14 mm or less.

[0102] The outer layer 92 covers the surface of the inner layer 91 that covers the front end surface 102e of the element body 102, and covers the inner layer 91 in the range of a predetermined length LB in the longitudinal direction of the element body 102 from the tip of the element body 102. The outer layer 92 preferably covers the inner layer 91 to such an extent that the structural strength of the entire porous protective layer 90 can be maintained. The outer layer 92 may completely cover the inner layer 91 (LB>LA), or a part including the rear end in the longitudinal direction of the inner layer 91 may be exposed as shown in FIG. 3 (LB<LA). Also, LB = LA may be possible. In the present embodiment, the longitudinal length LB of the outer layer 92 is the same on each of the two main surfaces (the pump surface 102a and the heater surface 102b) and the two side surfaces (the left surface 102c and the right surface 102d), but the longitudinal length LB of the outer layer 92 may be different from each other on the two main surfaces and the two side surfaces. Also, on the two main surfaces and the two side surfaces, the magnitude relationship (LB>LA, LB = LA, or LB<LA) between the longitudinal length LB of the outer layer 92 and the longitudinal length LA of the inner layer 91 may be different from each other.

[0103] The length LB of the outer layer 92 in the longitudinal direction from the tip of the element body 102 varies depending on the configuration of the element body 102, but may be, for example, 7 mm or more, 9 mm or more, or 10 mm or more. The length LB may also be, for example, 17 mm or less, or 14 mm or less. It is preferable that the length LB of the outer layer 92 in the longitudinal direction from the tip of the element body 102 is not too short compared to the length LA of the inner layer 91 in the longitudinal direction from the tip of the element body 102. This range is believed to maintain a higher structural strength of the porous protective layer 90 as a whole. For example, the length LB of the outer layer 92 in the longitudinal direction may be 80% or more, 85% or more, 90% or more, etc., of the length LA of the inner layer 91 in the longitudinal direction.

[0104] The length L of the entire porous protective layer 90 in the longitudinal direction from the tip of the element body 102 varies depending on the configuration of the element body 102, but may be, for example, 7 mm or more, 9 mm or more, or 10 mm or more. The length L may also be, for example, 17 mm or less, or 14 mm or less. In this embodiment, the longitudinal length L of the porous protective layer 90 is the same on each of the two main surfaces (the pump surface 102a and the heater surface 102b) and the two side surfaces (the left surface 102c and the right surface 102d), but the longitudinal length L of the porous protective layer 90 may be different from each other on the two main surfaces and the two side surfaces.

[0105] The thickness of the inner layer 91 may be, for example, 200 μm or more and 800 μm or less. In this embodiment, the thickness of the inner layer 91 of the porous protective layer 90 on each surface of the element body 102 is all approximately the same, but the thickness of the inner layer 91 on each surface of the element body 102 may be different. The thickness of the outer layer 92 may be, for example, 100 μm or more and 400 μm or less. In this embodiment, the thickness of the outer layer 92 of the porous protective layer 90 on each surface of the element body 102 is all approximately the same, but the thickness of the outer layer 92 on each surface of the element body 102 may be different.

[0106] The thickness is determined as follows using an image (SEM image) obtained by observation with a scanning electron microscope (SEM). The sensor element 101 is cut in the longitudinal direction of the sensor element 101 in a region where the element body 102 is present (for example, near the center in the width direction of the sensor element 101). The cut surface is filled with resin and polished to obtain an observation sample. The magnification of the SEM is set to 80 times, and the observation surface of the observation sample is photographed to obtain an SEM image of the cross section of the porous protective layer 90. The direction perpendicular to the surface of the element body 102 is defined as the thickness direction, and the distance from the surface of the outer layer 92 (the surface of the porous protective layer 90) to the interface with the inner layer 91 is calculated, and this distance is defined as the thickness of the outer layer 92. The distance from the surface of the inner layer 91 to the interface with the element body 102 is also calculated, and this distance is defined as the thickness of the inner layer 91. The distance from the surface of the outer layer 92 (the surface of the porous protective layer 90) to the interface with the element body 102 is also calculated, and this distance is defined as the thickness of the inner layer 91. The thickness of the porous protective layer 90 may be determined by calculating the sum of the thickness of the outer layer 92 and the thickness of the inner layer 91 thus derived.

[0107] 3, in the vicinity of the rear end in the longitudinal direction of the porous protective layer 90, the thickness of the inner layer 91 usually becomes thinner toward the rear end. In addition, the corners of the front end in the longitudinal direction of the porous protective layer 90 are often rounded. The above thickness is derived in a region where the porous protective layer 90 has a uniform thickness (a region other than the front and rear end portions).

[0108] Next, the porosity of the inner layer 91 of the porous protective layer 90 will be described. Fig. 4 is the same cross-sectional schematic view as Fig. 3, showing the configuration of the porous protective layer 90 (inner layer 91 and outer layer 92). In Fig. 4, as in Fig. 3, configurations other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode inside the element body 102 are omitted.

[0109] As described above, the inner layer 91 has the function of reducing thermal shock applied to the element body 102 due to its insulating effect. The higher the porosity of the inner layer 91, the lower the thermal conductivity of the inner layer 91 and the better the insulating effect. However, there is a concern that the higher the porosity of the inner layer 91, the lower the adhesion strength between the element body 102 and the inner layer 91. After extensive research, the inventors have found that by making the porosity of the inner layer 91 in the rear region 91c lower than the porosity of the inner layer 91 in the electrode presence region 91b, the insulating effect of the inner layer 91 can be improved while maintaining the adhesion strength between the element body 102 and the inner layer 91. In other words, by reducing the porosity of the inner layer 91 in the rear region 91c, the adhesion strength between the element body 102 and the rear region 91c of the inner layer 91 can be improved. By bonding the element body 102 and the inner layer 91 with sufficient strength in the rear region 91c of the inner layer 91, it is possible to maintain high bonding strength throughout the inner layer 91. In the electrode presence region 91b, a high heat insulating effect can be obtained due to the high porosity.

[0110] As described above, the porosity of the rear region 91c of the inner layer 91 is lower than the porosity of the electrode presence region 91b of the inner layer 91. That is, the inner layer 91 has different porosities in the longitudinal direction of the element body 102 (base portion 103). As shown in FIG. 4, this embodiment illustrates an example in which the porosity gradually decreases in the order of region A1, region A2, and region A3 from the front end of the element body 102 (base portion 103) toward the rear in the longitudinal direction. Region A1 is present in the front region 91a and the electrode presence region 91b, and regions A2 and A3 are present in the rear region 91c. At any position in the longitudinal direction of the rear region 91c of the inner layer 91, the porosity of the rear region 91c is lower than the porosity of the electrode presence region 91b.

[0111] Preferably, the porosity in the rear region 91c of the inner layer 91, in the longitudinal direction of the base portion 103, in a region farther from the one end (tip) of the base portion 103 in the longitudinal direction than the measurement gas flow space 15, is lower than the porosity in the electrode presence region of the inner layer.

[0112] The porosity of the rear region 91c of the inner layer 91 may be, for example, 30% by volume or more and 50% by volume or less. Within this range, the structural strength of the inner layer 91 can be maintained at a high level. Here, the porosity of the rear region 91c may be the average porosity of the rear region 91c. Alternatively, the porosity at a predetermined position in the longitudinal direction of the rear region 91c (for example, the center position) may be used as the porosity of the rear region 91c. The porosity of the rear region 91c of the inner layer 91 may be, for example, 30% by volume or more and 45% by volume or less, or 30% by volume or more and 40% by volume or less.

[0113] In the case where the porosity of the rear region 91c gradually decreases in the order of region A2 and region A3 as in the present embodiment, the porosity of the rearmost region A3 of the rear region 91c may be, for example, 30% to 50% by volume, more preferably 30% to 45% by volume, or 30% to 40% by volume.

[0114] The porosity of the electrode presence region 91b of the inner layer 91 may be, for example, 40% by volume or more and 80% by volume or less, provided that the porosity is higher than the porosity of the rear region 91c of the inner layer 91. Within this range, the heat insulating effect of the inner layer 91 can be maintained at a high level. Here, the porosity of the electrode presence region 91b may be the average porosity of the electrode presence region 91b. Alternatively, the porosity at a predetermined position (e.g., the center position) in the longitudinal direction of the electrode presence region 91b may be used as the porosity of the region. The porosity of the electrode presence region 91b of the inner layer 91 may be, for example, 40% by volume or more and 70% by volume or less, or 40% by volume or more and 60% by volume or less.

[0115] The porosity of the rear region 91c of the inner layer 91 may be 5% or more lower than the porosity of the electrode presence region 91b of the inner layer 91. Within this range, it is possible to maintain a high level of the structural strength of the inner layer 91 while maintaining a high level of the heat insulating effect of the inner layer 91. Furthermore, for example, the porosity of the rear region 91c of the inner layer 91 may be 6% or more lower than the porosity of the electrode presence region 91b of the inner layer 91.

[0116] The porosity of the front region 91a of the inner layer 91 is not particularly limited, but may be, for example, 30% by volume or more and 80% by volume or less. The porosity of the front region 91a of the inner layer 91 may be approximately the same as the porosity of the electrode presence region 91b of the inner layer 91, or may be higher or lower than the porosity of the electrode presence region 91b of the inner layer 91. The porosity may also be higher than the porosity of the rear region 91c. The porosity of the region of the inner layer 91 that covers the front end surface 102a of the element body 102 is not particularly limited, but may be, for example, approximately the same as the porosity of the front region 91a.

[0117] Here, a method for deriving the porosity will be described. Fig. 5 is the same cross-sectional schematic view as Fig. 3, showing the structure of the porous protective layer 90 (inner layer 91 and outer layer 92). In Fig. 5, as in Fig. 3, the structure inside the element body 102 other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode is omitted.

[0118] The porosity at a position Mp in the rear region 91c of the inner layer 91, which is a predetermined length Lp from the tip of the element body 102 in the longitudinal direction, may be used as the porosity in the rear region 91c of the inner layer 91. In FIG. 5, the position Mp of the predetermined length Lp is a position in the rear region 91c slightly behind the measurement gas flow space 15. In addition, the porosity at a position Me, which is a predetermined length Le from the tip of the element body 102 in the longitudinal direction, may be used as the porosity in the electrode presence region 91b. In FIG. 5, the position Mp of the predetermined length Lp is a position in the rear region 91c slightly behind the measurement gas flow space 15. Me is the central position of the electrode existing region 91b in the longitudinal direction.

[0119] The porosity is determined as follows using an image (SEM image) obtained by observation with a scanning electron microscope (SEM). As in the case of the thickness described above, an SEM image of the cross section of the inner layer 91 of the porous protective layer 90 is obtained by setting the SEM magnification to 80x. Specifically, the SEM image is obtained by photographing the rear region 91c of the inner layer 91, with the center at a position Mp a predetermined length Lp in the longitudinal direction from the tip of the element body 102. The SEM image shows an area of ​​the inner layer 91 measuring 500 μm in length and 1500 μm in width. The obtained SEM image is then binarized using "Otsu's binarization" (also known as discriminant analysis). In the binarized image, alumina is represented in white and pores in black. The area of ​​the alumina portion (white) and the area of ​​the pore portion (black) of the binarized image are obtained. The ratio of the area of ​​the pores to the total area (the sum of the area of ​​the alumina portion and the area of ​​the pores) may be calculated, and this value may be used as the porosity of the rear region 91c of the inner layer 91.

[0120] Also, an SEM image is obtained by photographing the electrode presence region 91b of the inner layer 91 so that the center is at a position Me that is a predetermined length Le from the tip of the element body 102 in the longitudinal direction. As in the case of the rear region 91c described above, the ratio of the area of ​​the pores to the total area (the sum of the area of ​​the alumina portion and the area of ​​the pores) is calculated for the obtained SEM image, and this value may be used as the porosity of the electrode presence region 91b of the inner layer 91.

[0121] The porosity of the rear region 91c of the inner layer 91 may be, for example, the porosity at the center of the rear region 91c. Alternatively, the porosity at the longitudinal rear end of the measurement gas flow space 15 may be used. Alternatively, the porosity at the center between the longitudinal rear end of the measurement gas flow space 15 and the rear end of the inner layer 91 may be used. Alternatively, the porosity at positions approximately two-thirds, three-quarters, and four-fifths of the longitudinal length LA of the inner layer 91 may be used as the porosity of the rear region 91c of the inner layer 91. Alternatively, the average value of porosities calculated at multiple positions may be used as the porosity of the rear region 91c of the inner layer 91.

[0122] Furthermore, the porosity of the electrode presence region 91b of the inner layer 91 may be, for example, the porosity at the center of the electrode presence region 91b as described above. Alternatively, the porosity may be at a position in the electrode presence region 91b where the temperature is high. Alternatively, using the longitudinal length LA of the inner layer 91 as a reference, the porosities at positions that are approximately 1 / 3, 1 / 4, or 2 / 5 of the length LA may be used as the porosity of the electrode presence region 91b of the inner layer 91. Alternatively, the average value of porosities calculated at multiple positions may be used as the porosity of the electrode presence region 91b of the inner layer 91.

[0123] As described above, the porosity of the outer layer 92 is lower than the porosity at any position in the inner layer 91. The porosity of the outer layer 92 may be, for example, approximately 10% by volume or more and 35% by volume or less, provided that the porosity is lower than the porosity at any position in the electrode presence region 91b and the rear region 91c of the inner layer 91. The porosity of the outer layer 92 can also be calculated using the porosity calculation method described above. It is considered that the outer layer 92 has substantially the same porosity regardless of the observation location. Therefore, a porosity value calculated using a single cross-sectional image may be used as the porosity value of the outer layer 92.

[0124] The sensor element 101 for detecting the NOx concentration in a measurement gas and the gas sensor 100 including the sensor element 101 have been described above as examples of embodiments of the present invention, but the present invention is not limited to these embodiments. The present invention may include sensor elements of various forms as long as they achieve the object of the present invention of improving the water resistance of the sensor element while maintaining the adhesion strength between the element body and the protective layer.

[0125] In the gas sensor 100 of the above-described embodiment, the porosity of the inner layer 91 of the porous protective layer 90 decreases stepwise from the tip of the element body 102 (base portion 103) to the rear in the longitudinal direction in the order of region A1, region A2, and region A3, as shown in Fig. 4, but is not limited to this. The inner layer 91 may have various structures as long as the porosity of the rear region 91c of the inner layer 91 is lower than the porosity of the electrode-existing region 91b.

[0126] For example, the inner layer 91 may have different porosities in the region on the front end side and the region on the rear end side, in the longitudinal direction of the element body 102, with the rear end of the electrode presence region 91b, i.e., the front end of the rear region 91c as the boundary (porosity in the region on the front end side > porosity in the region on the rear end side).

[0127] Another example of the porous protective layer is shown in FIG. 6. FIG. 6 is a cross-sectional view, taken along the same line as FIGS. 3 to 5, illustrating the configuration of a porous protective layer 290 (inner layer 291 and outer layer 92) of another example of a sensor element 201 having a different porous protective layer configuration. Similar to FIG. 3, FIG. 6 does not illustrate components other than the measurement gas flow space 15, the space inner electrode, and the space outer electrode inside the element body 102. In FIG. 6, the same components as those in FIGS. 3 to 4 are denoted by the same reference numerals. The inner layer 291 of the porous protective layer 290 has different porosities in a front-end region A11 and a rear-end region A12, separated by the rear end of the measurement gas flow space 15 in the longitudinal direction of the element body 102 (the porosity of the front-end region A11 is greater than the porosity of the rear-end region A12). That is, the inner layer 291 (porous protective layer 290) is configured so that a high-porosity region A11 exists in the void region where the measurement gas flow void 15 exists, and a low-porosity region A12 exists in the rearmost region following the void region. That is, in the sensor element 201, the porosity of the rearmost region following the electrode region, where the measurement gas flow void 15 does not exist, is lower than the porosity of the electrode region.

[0128] Furthermore, for example, when the inner layer 91 is divided into three equal parts, a front part, a middle part, and a rear part, based on the longitudinal length LA of the inner layer 91 from the tip of the element body 102, the porosity of the rear part may be lower than the porosity of the middle part. In this case, the porosity of the front part may be approximately the same as the porosity of the middle part, or may be higher or lower than the porosity of the middle part. Also, the porosity of the front part may be higher than the porosity of the rear part.

[0129] The porosity of the inner layer 91 may decrease stepwise or continuously from the one longitudinal end (tip) of the element body 102 (base portion 103) along the longitudinal direction of the element body 102 (base portion 103). In the above-described embodiment, the porosity of the inner layer 91 decreases in three steps, but the porosity may decrease stepwise (approximately stepwise) in four or more steps. Furthermore, the porosity may decrease continuously (approximately continuously).

[0130] In the gas sensor 100 of the above embodiment, the porous protective layer 90 is composed of the inner layer 91 and the outer layer 92, but this is not limiting. One or more intermediate layers may be present between the inner layer 91 and the outer layer 92.

[0131] In the gas sensor 100 of the above-described embodiment, the outer pump electrode 23 is located on the surface of the element body 102, and the inner layer 91 of the porous protective layer 90 is formed in contact with the outer pump electrode 23 on the surface of the element body 102. However, this is not limiting. For example, the element body 102 may have main surface protective layers on its two main surfaces (the pump surface 102a and the heater surface 102b). The main surface protective layers are provided to prevent foreign matter and poisonous substances from adhering to the main surfaces (the pump surface 102a and the heater surface 102b) of the element body 102 and the outer pump electrode 23 on the pump surface 102a. The main surface protective layer is a porous layer made of ceramics such as alumina. The porosity of the main surface protective layer may be, for example, approximately 20% by volume or more and 40% by volume or less. The thickness of the main surface protective layer may be, for example, approximately 5 μm to 30 μm.

[0132] Furthermore, for example, the element body 102 may further be provided with an underlayer when the inner layer 91 is formed. The underlayer is provided to further improve adhesion between the element body 102 and the inner layer 91. When an underlayer is provided, it is preferable to provide it on at least two main surfaces (the pump surface 102a and the heater surface 102b) of the element body 102. The underlayer is, for example, a porous layer made of ceramics such as alumina. The porosity of the underlayer may be approximately 40% by volume or more. For example, it may be approximately 40% by volume or more and 60% by volume or less. The thickness of the underlayer may be, for example, approximately 20 μm to 60 μm.

[0133] In the above-described embodiment, the gas sensor 100 detects the NOx concentration in the measurement gas, but the measurement target gas is not limited to NOx. The sensor element of the gas sensor 100 may be configured using an oxygen ion conductive solid electrolyte. The measurement target gas may be, for example, an oxide gas other than oxygen O or NOx (e.g., carbon dioxide CO, water HO, etc.). Alternatively, it may be a non-oxide gas such as ammonia NH.

[0134] In the gas sensor 100 of the above-described embodiment, the sensor element 101 has three internal cavities, namely, the first internal cavities 20, the second internal cavities 40, and the third internal cavities 61, and the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 are respectively disposed in the internal cavities, as shown in FIG. 2 . However, the present invention is not limited to this. For example, the sensor element 101 may have two internal cavities, namely, the first internal cavities 20 and the second internal cavities 40, and the inner main pump electrode 22 is disposed in the first internal cavities 20, and the auxiliary pump electrode 51 and the measurement electrode 44 are disposed in the second internal cavities 40. In this case, for example, a porous protective layer covering the measurement electrode 44 may be formed as a diffusion rate-controlling part between the auxiliary pump electrode 51 and the measurement electrode 44. Furthermore, for example, the number of internal cavities may be one or four or more.

[0135] In the gas sensor 100 according to the above embodiment, the outer pump electrode 23 functions as three electrodes: the outer main pump electrode in the main pump cell 21, the outer auxiliary pump electrode in the auxiliary pump cell 50, and the outer measurement electrode in the measurement pump cell 41. However, this is not limiting. For example, the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may each be formed as separate electrodes. For example, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided separately from the outer pump electrode 23 on the outer surface of the base portion 103 so as to be in contact with the measurement gas. In this case, the electrode presence region refers to the region extending in the longitudinal direction of the element body 102 from the tip of the most forwardly disposed electrode among the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode to the rear end of the most rearwardly disposed electrode. In the longitudinal direction of the element body 102, the rear end of the electrode presence region is usually located closer to the tip of the measurement gas flow space 15 or at the same level as the rear end of the measurement gas flow space 15.

[0136] In addition to the above configuration, the components of the element body 102, such as the measurement target gas flow space 15 and the electrodes, can take various forms depending on the type of gas to be measured, the purpose and environment of use of the gas sensor, etc.

[0137] [Sensor element manufacturing method] Next, an example of a method for manufacturing the above-described sensor element will be described. In the method for manufacturing the sensor element 101, first, the element body 102 is manufactured, and then the porous protective layer 90 is formed on the element body 102, thereby manufacturing the sensor element 101.

[0138] The following description will be given taking as an example the case of manufacturing a sensor element 101 made up of six layers as shown in Fig. 2. Fig. 7 is a flowchart showing an example of a method for manufacturing a sensor element.

[0139] (Manufacturing of the element body) First, a method for manufacturing the element body 102 will be described. First, six blank sheets are prepared (step S1). The blank sheets are green sheets containing an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component. A known forming method can be used to manufacture the green sheets. Sheet holes and the like used for positioning during printing and lamination are formed in advance in each of the six green sheets by a known method such as punching with a punching device. These are referred to as blank sheets. Note that in the blank sheet used for the spacer layer 5, through-holes such as internal voids are also formed by a similar method. Necessary through-holes are also formed in the other layers in advance. All six blank sheets may be the same thickness, or the thickness may vary depending on the layer being formed.

[0140] Various patterns required for each layer are printed and dried on blank sheets to be used for the six layers, namely, 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 (step S2). A known screen printing technique can be used to print the patterns. A known drying means can also be used for the drying process.

[0141] These steps are repeated until various patterns have been printed and dried on each of the six blank sheets. The six printed blank sheets are then stacked in a predetermined order while being positioned using sheet holes, etc., and subjected to a bonding process in which they are bonded under predetermined temperature and pressure conditions to form a laminate (step S3). The bonding process is carried out by applying heat and pressure using a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressing depend on the laminator used, but can be determined appropriately to achieve good lamination.

[0142] The obtained laminate contains a plurality of element bodies 102. The laminate is cut into units of element bodies 102 (step S4). The cut laminate is fired at a predetermined firing temperature to obtain element bodies 102 (step S5). The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 103 of the sensor element 101 is sintered to form a dense body and at which the electrodes and the like maintain a desired porosity. For example, firing is performed at a firing temperature of about 1300 to 1500°C.

[0143] (Protective layer manufacturing) Next, a description will be given of a method for forming the porous protective layer 90 (the inner layer 91 and the outer layer 92) on the element body 102. In this embodiment, the porous protective layer 90 is formed by plasma spraying.

[0144] First, a sprayed film that will become the inner layer 91 is formed. That is, a powder for forming the inner layer that has been prepared in advance is sprayed onto a predetermined region of the element body 102 (step S6). Next, a powder for forming the outer layer that has been prepared in advance is sprayed onto a predetermined region on the sprayed film that will become the inner layer 91 of the element body 102 (step S7). Thereafter, degreasing is performed (step S8), and the porous protective layer 90 (inner layer 91 and outer layer 92) is formed.

[0145] The powder for forming the inner layer contains a raw material powder (alumina powder in this embodiment) made of the material of the inner layer 91 and having a predetermined particle size distribution, and a pore-forming material. Examples of the pore-forming material that can be used include xanthine derivatives such as theobromine, organic resin materials such as acrylic resins, and inorganic materials such as carbon. The powder for forming the inner layer contains alumina powder and organic pore-forming material in a ratio corresponding to the porosity of the inner layer 91 to be formed. In this embodiment, powders for forming the inner layer are prepared according to the porosity of each of the regions A1, A2, and A3 of the inner layer 91.

[0146] The powder for forming the inner layer is sprayed onto the region of the surface of the element body 102 where the inner layer 91 is to be formed, thereby forming a sprayed film (step S6). A known plasma spraying technique can be used for the spraying. Specifically, a sprayed film corresponding to the porosity of region A1 is formed on the region where region A1 is to be formed among the regions where the inner layer 91 is to be formed. A sprayed film corresponding to the porosity of region A2 is formed on the region where region A2 is to be formed among the regions where the inner layer 91 is to be formed. A sprayed film corresponding to the porosity of region A3 is formed on the region where region A3 is to be formed among the regions where the inner layer 91 is to be formed. The order in which these sprayed films are formed can be determined as appropriate. Furthermore, multiple regions may be formed simultaneously.

[0147] Next, the outer layer 92 is formed. The outer layer 92 is formed by spraying a powder for forming the outer layer prepared in advance onto a predetermined region on the sprayed film that is to become the inner layer 91 of the element body 102 (step S7).

[0148] The powder for forming the outer layer contains raw material powder (alumina powder in this embodiment) made of the material of the outer layer 92 and having a predetermined particle size distribution. Unlike the powder for forming the inner layer, the powder for forming the outer layer does not usually contain a pore-forming material.

[0149] The powder for forming the outer layer is sprayed onto an area where the outer layer 92 is to be formed on the surface of the sprayed film that will become the inner layer 91 formed on the element body 102, thereby forming the sprayed film (i.e., the outer layer 92) (step S7). A known plasma spraying technique can be used for the spraying.

[0150] Finally, the sprayed film that will become the inner layer 91 is heat-treated and degreased (step S8). By degreasing, the pore-forming material in the sprayed film is removed, and the inner layer 91 is formed as a porous body. The degreasing process is carried out at a predetermined degreasing temperature. The degreasing temperature may be any temperature at which all of the pore-forming material components in the film of the inner layer 91 are removed and the porous structure of the inner layer 91 is maintained. The degreasing temperature may be lower than the firing temperature of the element body 102. For example, degreasing is carried out at a degreasing temperature of about 400 to 900°C. During the degreasing process, the sprayed film of the outer layer 92, which does not contain the pore-forming material, is also heat-treated at the same time.

[0151] In the above manufacturing method, the thermal spraying of the powder for forming the inner layer (step S6), the thermal spraying of the powder for forming the outer layer (step S7), and degreasing (step S8) are carried out in that order, but it is also possible to carry out the thermal spraying of the powder for forming the inner layer (step S6) and degreasing (step S8) first, and then the thermal spraying of the powder for forming the outer layer (step S7).

[0152] In the above manufacturing method, the inner layer 91 and the outer layer 92 are formed by plasma spraying, but this is not limiting. The inner layer 91 and the outer layer 92 may be formed by other methods such as screen printing, dipping, gel casting, etc. Furthermore, the inner layer 91 and the outer layer 92 may be formed by different methods.

[0153] The obtained sensor element 101 is housed in a predetermined housing and assembled into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas. [Example]

[0154] Examples in which sensor elements were specifically fabricated and tested will be described below as examples, but the present invention is not limited to the following examples.

[0155] [1. Preparation of Examples 1 to 3 and Comparative Example 1] The sensor elements of Examples 1 to 3 were fabricated according to the manufacturing method of the sensor element 101 described above. Specifically, first, an element body 102 was fabricated having a length of 67.5 mm in the front-rear direction, a width of 4.25 mm in the left-right direction, and a thickness of 1.45 mm in the up-down direction. The outer pump electrode 23 was disposed 3.1 mm rearward from a position 1.6 mm longitudinally from the tip of the element body 102. Thereafter, the porous protective layer 90 (inner layer 91 and outer layer 92) was formed by plasma spraying.

[0156] In all of the sensor elements of Examples 1 to 3, the porosity of the inner layer 91 gradually decreased in the longitudinal direction from the tip of the element body 102. In all of the sensor elements of Examples 1 to 3, the length of the inner layer 91 in the longitudinal direction from the tip of the element body 102 was 10 mm, and the thickness of the inner layer 91 was 500 μm. In all of the sensor elements of Examples 1 to 3, the length of the outer layer 92 in the longitudinal direction from the tip of the element body 102 was 9 mm, and the thickness of the outer layer 92 was 200 μm. The porosity of the outer layer 92 was 25%.

[0157] The sensor element of Comparative Example 1 was produced in the same manner as the sensor elements of Examples 1 to 3, except that the porosity of the inner layer 91 was gradually increased in the longitudinal direction from the tip of the element body 102. In the sensor element of Comparative Example 1, the length LA of the inner layer 91 in the longitudinal direction from the tip of the element body 102 was 10 mm, and the thickness of the inner layer 91 was 500 μm.

[0158] 5, in the sensor elements of Examples 1 to 3 and Comparative Example 1, the porosity in the electrode region 91b was determined from an SEM image taken at a position Me 4 mm (Le = 4 mm) longitudinally from the tip of the element body 102. That is, it was determined from an SEM image taken at a position Me 2 / 5 (Le = 2LA / 5) of the longitudinal length LA of the inner layer 91 in the longitudinal direction from the tip of the element body 102. In addition, in the sensor elements of Examples 1 to 3 and Comparative Example 1, the porosity in the rear region 91c was determined from an SEM image taken at a position Mp 8 mm (Lp = 8 mm) longitudinally from the tip of the element body 102. That is, it was determined from an SEM image taken at a position Mp 4 / 5 (Lp = 4LA / 5) of the longitudinal length LA of the inner layer 91 in the longitudinal direction from the tip of the element body 102. The porosity in the electrode region 91b and the porosity in the rear region 91c were each determined using the above-described method for deriving porosity. The porosity difference [= (porosity in the electrode presence region 91b) - (porosity in the rear region 91c)] was calculated. A positive porosity difference indicates that the porosity in the rear region 91c is lower than the porosity in the electrode presence region 91b. Hereinafter, the unit of porosity (%) means volume %.

[0159] In the sensor element of Example 1, the porosity in the electrode existing region 91b was 55.7%, the porosity in the rear region 91c was 54.1%, and the porosity difference was 1.6%.

[0160] In the sensor element of Example 2, the porosity in the electrode existing region 91b was 42.7%, the porosity in the rear region 91c was 37.1%, and the porosity difference was 5.6%.

[0161] In the sensor element of Example 3, the porosity in the electrode existing region 91b was 56%, the porosity in the rear region 91c was 37.8%, and the porosity difference was 18.2%.

[0162] In the sensor element of Comparative Example 1, the porosity in the electrode existing region 91b was 45.7%, the porosity in the rear region 91c was 56.1%, and the porosity difference was −10.4%.

[0163] [2. Evaluation of water resistance] The performance of the porous protective layer 90 (water resistance of the sensor element 101) was evaluated for the sensor elements 101 of Examples 1 to 3 and Comparative Example 1. Specifically, the heater 72 was first energized to heat the sensor element 101 to a temperature of 800°C. In this state, the main pump cell 21, the auxiliary pump cell 50, the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump were operated in the air atmosphere to control the oxygen concentration in the first internal space 20 to maintain a predetermined constant value. After waiting for the pump current Ip0 to stabilize, a water droplet was dropped on the porous protective layer 90 on the pump surface 102a at approximately the center of the electrode presence region in the longitudinal direction. The presence or absence of a crack in the sensor element 101 was determined based on whether the pump current Ip0 increased by 5% or more. If a crack occurs in the sensor element 101 due to the thermal shock caused by the water droplets, oxygen will be more likely to flow through the crack into the first internal space 20, increasing the value of the pump current Ip0. Therefore, if the pump current Ip0 increases by 5% or more, it is determined that a crack has occurred in the sensor element 101 due to the water droplets.

[0164] Additionally, the amount of water droplets was gradually increased up to 60 μL, and multiple tests were performed to determine the amount of water droplets when the pump current Ip0 increased by 5% or more (when a crack was suspected to have occurred in the sensor element 101). Twelve sensor elements 101 for each of Examples 1 to 3 and Comparative Example 1 were prepared, and the average value of the water droplet amounts for the 12 elements was calculated for each of Examples 1 to 3 and Comparative Example 1. This average water droplet amount was used as an index of water resistance. A larger average water droplet amount indicates higher water resistance.

[0165] [3. Evaluation of pull-out strength] For the sensor elements 101 of Examples 1 to 3 and Comparative Example 1, the pull-out strength of the porous protective layer 90 from the element body 102 was evaluated as an index of the structural strength of the porous protective layer 90. The pull-out strength was measured as follows. The porous protective layer 90 of the sensor element 101 was fixed in a test jig. Specifically, a test jig was prepared having a through-hole of approximately the same size as a cross section perpendicular to the longitudinal direction of the element body 102 of the sensor element 101. The sensor element 101 was passed through the through-hole of the test jig so that the porous protective layer 90 was caught in the through-hole of the test jig. In this state, the test jig and the rear end side of the sensor element 101 were pulled using a tensile tester, and a stress-strain curve (SS curve) was measured. An autograph testing machine (AGS-5kNx; manufactured by Shimadzu Corporation) was used as the tensile tester. The maximum stress required to deform 1 mm was calculated from the obtained stress-strain curve (SS curve). Twelve sensor elements 101 were prepared for each of Examples 1 to 3 and Comparative Example 1, and the average value of the maximum stress of the 12 elements was calculated for each of Examples 1 to 3 and Comparative Example 1. The average value of the maximum stress was defined as the pull-out strength of the porous protective layer 90.

[0166] Table 1 shows the results of water resistance and pull-out strength.

[0167] [Table 1]

[0168] As shown in Table 1, it was confirmed that Examples 1 to 3 were all able to maintain both water resistance and pull-out strength at the same level or higher than Comparative Example 1. It was also confirmed that Examples 2 and 3 maintained high water resistance while clearly improving pull-out strength.

[0169] As described above, according to the present invention, the rear region 91c of the inner layer 91 can maintain high adhesive strength between the element body 102 and the inner layer 91, while maintaining high heat insulating effect of the electrode presence region 91b. Therefore, it is possible to provide a sensor element that has high water resistance while maintaining high adhesive strength between the element body 102 and the porous protective layer 90. [Explanation of symbols]

[0170] 1 First board layer 2 Second board layer 3 Third board 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 15 Measurement gas flow space 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a (Inner main pump electrode) ceiling electrode part 22b Bottom electrode part (of inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (for main pump cell) 30 Third diffusion-controlled section 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 46 Variable power supply (for measuring pump cell) 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a (auxiliary pump electrode) ceiling electrode part 51b (auxiliary pump electrode) bottom electrode part 52 Variable power supply (for auxiliary pump cell) 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 71 Heater electrode 72 Heater 73 through holes 74 Heater insulation layer 75 Pressure relief hole 76 Heater lead 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, 290 porous protective layer 91, 291 inner layer 92 Outer layer 100 Gas Sensor 101, 201 sensor element 102 Element body 103 Base

Claims

1. an element body including a long, plate-like substrate including an oxygen ion conductive solid electrolyte layer and a measurement gas flow space formed on one end of the substrate in the longitudinal direction; a porous protective layer formed from the one longitudinal end of the base portion and covering a predetermined length of the surface of the element body in the longitudinal direction; A sensor element comprising: the element body includes an intra-space electrode disposed in the measurement gas flow space, and an outer-space electrode having a predetermined length in the longitudinal direction of the base portion and disposed on one of two main surfaces of the element body, corresponding to the intra-space electrode; the protective layer includes an inner layer covering the element body and an outer layer located outside the inner layer, and has, in the longitudinal direction of the base portion, an electrode presence region in which the outer-space electrodes are present, and a rear region following the electrode presence region; the porosity of the inner layer in the rear region is lower than the porosity of the inner layer in the electrode presence region; A sensor element for detecting a measurement target gas in a measurement gas, wherein the porosity of the outer layer is lower than the porosity of the rear region of the inner layer.

2. 2. The sensor element according to claim 1, wherein the rear region of the protective layer extends in the longitudinal direction of the base portion to a position farther from the one end of the base portion in the longitudinal direction than the measurement gas flow space.

3. 3. The sensor element according to claim 2, wherein the porosity of the rear region of the protective layer in a region farther from the one end of the base portion in the longitudinal direction than the measurement gas flow space is lower than the porosity of the electrode presence region of the inner layer.

4. 2. The sensor element according to claim 1, wherein the porosity of the rear region of the inner layer is equal to or greater than 30% by volume and equal to or less than 50% by volume.

5. 2. The sensor element according to claim 1, wherein the porosity of the inner layer in the electrode presence region is 40% by volume or more and 80% by volume or less, provided that the porosity is higher than the porosity of the inner layer in the rear region.

6. 2. The sensor element according to claim 1, wherein the porosity of the rear region of the inner layer is at least 5% lower by volume than the porosity of the electrode-containing region of the inner layer.

7. the rear region of the protective layer extends in the longitudinal direction of the base portion to a position far from the one end of the measurement target gas flow space in the longitudinal direction of the base portion, 2. The sensor element according to claim 1, wherein the porosity of the rear region of the inner layer is at least 5% lower than the porosity of the electrode-containing region of the inner layer.

8. 2. The sensor element according to claim 1, wherein the porosity of the inner layer decreases stepwise (including one step or two or more steps) or continuously from the one end in the longitudinal direction of the base portion along the longitudinal direction of the base portion.

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