Gas sensor

Abstract

To provide a gas sensor capable of precisely measuring a measuring object gas even in a measured gas with coexisting gaseous species that may generate exothermic reaction.SOLUTION: A gas sensor 100 comprises a long-sized tabular sensor element 101, and a bottomed cylindrical protection cover 130 accommodating part of the sensor element 101 including one end thereof. A ratio La / Lb of length La from the one end of the sensor element 101 to a longer direction midpoint of a gas inflow port H2 penetrating a side surface of the protection cover 130, in a longer direction of the sensor element 101 to length index Lb between the sensor element 101 on a flat surface perpendicular to the longer direction of the sensor element 101 including the longer direction midpoint of the gas inflow port H2 and a circle defined by an inner surface of the protection cover 130 is 0.9 or more. A ratio S / V of a surface area S of a length La part of two main surfaces and two side surfaces of the sensor element 101 to space volume V present in a length La part of the protection cover 103 is 0.15 mm-1 or more.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a gas sensor for detecting a target gas contained in a measurement target 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 measurement gases such as automobile exhaust gas. For example, gas sensors are installed in the exhaust path of gasoline or diesel vehicles and are used to measure the concentration of nitrogen oxides NOx, ammonia NH3, etc. in the exhaust gas.

[0003] As such a gas sensor, a gas sensor having a sensor element using an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) and a protective cover that protects a portion of the sensor element that contacts the measured gas is known. In such gas sensors, protective covers of various structures are used according to various purposes such as reducing water splash on the sensor element and reducing temperature fluctuation of the sensor element due to the measured gas (for example, Japanese Patent No. 3829026, Japanese Patent No. 4174004, Japanese Patent No. 6998802, Japanese Patent Publication No. 2021-148656, and Japanese Patent No. 4571974). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3829026 [Patent Document 2] Patent No. 4174004 [Patent Document 3] Patent No. 6998802 [Patent Document 4] Patent Publication No. 2021-148656 [Patent Document 5] Patent No. 4571974 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a demand to measure the concentrations of nitrogen oxides (NOx) and ammonia (NH3) in exhaust gas not only from gasoline and diesel vehicles, but also from hydrogen engine vehicles.

[0006] It is assumed that the exhaust path of a hydrogen engine vehicle will be a gas atmosphere in which oxygen and hydrogen coexist. It is widely known that hydrogen explosions can occur when oxygen and hydrogen are within a certain concentration range, and that hydrogen will spontaneously ignite when exposed to high temperatures of 500°C or higher in the atmosphere.

[0007] When a gas sensor as described above is used in the exhaust path of a hydrogen engine vehicle, depending on the gas atmosphere in the exhaust path (particularly the oxygen concentration and hydrogen concentration) and the operating conditions such as the temperature of the gas sensor, oxygen and hydrogen may react suddenly in the gas sensor, and the thermal stress resulting from the reaction heat may cause cracks in the internal structure of the sensor element.

[0008] Therefore, an object of the present invention is to provide a gas sensor that can suppress the occurrence of cracks in the sensor element and accurately measure the target gas in the measured gas, even when a gas containing coexisting gas species that can cause an exothermic reaction, such as a gas containing coexisting oxygen and hydrogen, is introduced into the gas sensor as the measured gas. [Means for solving the problem]

[0009] As a result of extensive investigation, the inventors have found that by specifying the positional relationship between the sensor element and a gas inlet provided in a protective cover that houses a part of the sensor element as follows, it is possible to suppress the occurrence of cracks in the sensor element even when a gas containing coexisting gas species that can cause an exothermic reaction is introduced into the gas sensor as the measured gas.

[0010] The present invention includes the following inventions.

[0011] (1) A long plate-like sensor element including an oxygen ion conductive solid electrolyte layer, the sensor element having a measurement gas flow space formed on one end side in the longitudinal direction of the sensor element; a bottomed cylindrical protective cover that accommodates a portion of the sensor element including the one end, the protective cover has a gas inlet penetrating a side surface of the protective cover surrounding the sensor element, The length of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the gas inlet of the protective cover in the longitudinal direction is represented as length La, In a plane perpendicular to the longitudinal direction of the sensor element and including the longitudinal midpoint of the gas inlet of the protective cover, the longest distances between each of the two main surfaces of the sensor element and a circumference of a circle defined by the inner surface of the protective cover that faces each of the main surfaces are defined as length x1 and length x2, and the longest distances between each of the two side surfaces of the sensor element and a circumference of the circle that faces each of the side surfaces are defined as length y1 and length y2, respectively, and an index of the length between the sensor element and the circle defined by the inner surface of the protective cover is expressed as length index Lb=(x1+x2+y1+y2) / 4, The ratio La / Lb of the length La to the length index Lb is 0.9 or more, and When the total surface area of ​​the two main surfaces and the two side surfaces of the sensor element in the longitudinal direction of the length La is expressed as a surface area S, and the volume of the space existing in the longitudinal direction of the length La in the protective cover is expressed as a volume V, the ratio S / V of the surface area S to the volume V is 0.15 mm -1 The gas sensor for detecting a measurement target gas in a measurement gas is as described above.

[0012] (2) The gas sensor according to (1) above, wherein the length La in the longitudinal direction is 10 mm or less.

[0013] (3) The gas sensor according to (1) or (2), wherein the length x1 and the length x2 satisfy x1=x2=x, the length y1 and the length y2 satisfy y1=y2=y, and the length index Lb is expressed as Lb=(x+y) / 2.

[0014] The above (3) refers to the case where the sensor element is disposed at the center of the protective cover in a plane perpendicular to the longitudinal direction of the sensor element and including the longitudinal midpoint of the gas inlet of the protective cover.

[0015] (4) The gas sensor according to any one of (1) to (3) above, wherein the protective cover has a gas outlet port at a bottom portion.

[0016] (5) An outer cover for accommodating the protective cover is disposed on the outside of the protective cover, The gas sensor according to any one of (1) to (4) above, wherein the outer cover has an outer gas inlet at a position different from the gas inlet of the protective cover in the longitudinal direction of the sensor element.

[0017] (6) The gas sensor according to (5) above, wherein the outer cover has an outer gas outlet at a position different in the longitudinal direction of the sensor element from the gas inlet of the protective cover and the outer gas inlet of the outer cover.

[0018] (7) The gas sensor according to any one of (1) to (6) above, wherein the measurement gas flow space of the sensor element has a gas inlet opening at an end face of the one end in the longitudinal direction. Effect of the Invention

[0019] By using the gas sensor of the present invention, even when a gas containing coexisting gas species that can cause an exothermic reaction, such as a gas containing coexisting oxygen and hydrogen, is introduced into the gas sensor as the measured gas, it is possible to provide a gas sensor that can suppress the occurrence of cracks in the sensor element and accurately measure the target gas in the measured gas. [Brief description of the drawings]

[0020] [Figure 1] 1 is a schematic partial cross-sectional view showing an example of a schematic configuration of a gas sensor 100. FIG. [Diagram 2] 1 is a schematic vertical cross-sectional view in the longitudinal direction of a sensor element 101, showing an example of a schematic configuration of a gas sensor 100. FIG. [Diagram 3] 2 is a conceptual diagram showing the flow of a measurement gas in an element protective cover 120 of the gas sensor 100. FIG. [Figure 4] 2 is a schematic partial cross-sectional view similar to FIG. 1 for explaining the length La. FIG. [Diagram 5] 5 is an enlarged schematic cross-sectional view taken along line VV in Fig. 4 for explaining the length index Lb. In Fig. 5, the outer protective cover 140 is omitted from the illustration. [Figure 6] 2 is a schematic partial cross-sectional view similar to FIG. 1 for explaining the volume V. FIG. [Figure 7] 7 is an enlarged schematic cross-sectional view taken along line VII-VII in FIG. 6 for explaining surface area S and volume V. FIG. [Figure 8] FIG. 2 is a perspective view of a sensor element 101 for explaining a surface area S. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The gas sensor of the present invention includes a sensor element and a bottomed, tubular protective cover that accommodates a portion of the sensor element including one end thereof.

[0022] The sensor element included in the gas sensor of the present invention is in the form of a long plate including an oxygen ion conductive solid electrolyte layer, and a measurement gas flow space is formed on one end side in the longitudinal direction of the sensor element.

[0023] The gas sensor according to the present invention includes a protective cover that is a cylindrical cover with a bottom and that accommodates a part of the sensor element including the one end where the measurement gas flow space is formed. The protective cover has a gas inlet penetrating a side surface of the protective cover that surrounds the sensor element.

[0024] In the gas sensor of the present invention, The length of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the gas inlet of the protective cover in the longitudinal direction is represented as length La, In a plane perpendicular to the longitudinal direction of the sensor element and including the longitudinal midpoint of the gas inlet of the protective cover, the longest distances between each of the two main surfaces of the sensor element and a circumference of a circle defined by the inner surface of the protective cover that faces each of the main surfaces are defined as length x1 and length x2, and the longest distances between each of the two side surfaces of the sensor element and a circumference of the circle that faces each of the side surfaces are defined as length y1 and length y2, respectively, and an index of the length between the sensor element and the circle defined by the inner surface of the protective cover is expressed as length index Lb=(x1+x2+y1+y2) / 4, The ratio La / Lb of the length La to the length index Lb is 0.9 or more, and When the total surface area of ​​the two main surfaces and the two side surfaces of the sensor element in the longitudinal direction of the length La is expressed as a surface area S, and the volume of the space existing in the longitudinal direction of the length La in the protective cover is expressed as a volume V, the ratio S / V of the surface area S to the volume V is 0.15 mm -1 That's all.

[0025] [Outline of gas sensor configuration] The gas sensor of the present invention will be described below with reference to the drawings. FIG. 1 is a partial cross-sectional view showing an example of the schematic configuration of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a vertical cross-sectional view of a sensor element 101 in the longitudinal direction, showing an example of the schematic configuration of the gas sensor 100. The internal structure of the sensor element 101 is shown in detail in FIG. 2, and the internal structure of the sensor element 101 is omitted in FIG. 1 except for a part of it. In the following, with reference to FIG. 2, the upper side of FIG. 2 is the upper side, the lower side is the lower side, the left side of FIG. 2 is the front side, and the back side is the left side. In FIG. 1, in correspondence with FIG. 2, the front side perpendicular to the paper surface of FIG. 1 is the upper side, and the back side is the lower side, the lower side of FIG. 1 is the front side, the upper side is the rear side, the right side is the right, and the left side is the left.

[0026] 1, the gas sensor 100 includes a sensor element 101 and an element protective cover 120 that protects the tip side of the sensor element 101. The gas sensor 100 also includes an element sealing body 150 that seals and fixes the sensor element 101, and a fixing portion 160 attached to the element sealing body 150. The gas sensor 100 is attached to a pipe such as an exhaust gas pipe of a vehicle, and is used to measure the concentration of a measurement target gas such as NOx, NH3, or O2 contained in exhaust gas as a measurement target gas. In this embodiment, the gas sensor 100 measures the concentration of NOx as a measurement target gas.

[0027] The element sealing body 150 includes a cylindrical housing 151 made of metal and a ceramic supporter 152 sealed in a through hole inside the housing 151. The sensor element 101 is located on the central axis of the element sealing body 150 and penetrates the element sealing body 150 in the front-rear direction. A green compact (not shown) made of ceramic powder such as talc and at least one ceramic supporter (not shown) are arranged on the rear end side of the sensor element 101 from the ceramic supporter 152 in the through hole inside the housing 151. The green compact sandwiched between the ceramic supporters seals the through hole inside the housing 151, ensuring airtightness between the front end side and the rear end side of the sensor element 101 and fixing the sensor element 101.

[0028] The fixing portion 160 is a doughnut-shaped metal member. The fixing portion 160 is fixed coaxially to the housing 151, and has a male thread (not shown) formed on its outer circumferential surface. The gas sensor 100 is screwed to the piping by the fixing portion 160. The fixing portion 160 may be configured to be fixed by welding or the like. The fixing portion 160 may also be formed integrally with the element sealing body 150. The fixing portion 160 fixes the gas sensor 100 to the piping in a state in which the tip of the sensor element 101 and a portion of the element protective cover 120 of the gas sensor 100 protrude into the piping.

[0029] The rear end side of the sensor element 101 is accommodated in a metal outer cylinder (not shown) that is welded to the housing 151. The space inside the outer cylinder is filled with a reference gas (e.g., air) when measuring the NOx concentration, and the rear end side of the sensor element 101 is in contact with the reference gas. Each electrode of the sensor element 101 is electrically connected to the outside (such as a power source or a control device) via a connector arranged at the rear end of the sensor element 101.

[0030] The element protective cover 120 includes a cylindrical inner protective cover 130 with a bottom that accommodates a part of the sensor element 101 including the tip, and a cylindrical outer protective cover 140 with a bottom that accommodates the inner protective cover 130. The element protective cover 120 has a plurality of through holes for allowing gas to flow therethrough, and is configured to supply the measurement gas to the tip side of the sensor element 101 through the through holes. Details of the element protective cover 120 will be described later.

[0031] (sensor element) As shown in FIG. 2, the sensor element 101 is a long plate-like element including a base portion 102 having a structure in which a plurality of oxygen ion conductive solid electrolyte layers are laminated. The long plate-like shape is also referred to as a long plate shape or a strip shape. The base portion 102 has a structure in which six layers, namely 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 (ZrO2), are laminated in this order from the bottom as viewed in the drawing. The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness, or each layer may have a different thickness. The layers are bonded to each other via an adhesive layer made of a solid electrolyte, and the base portion 102 includes the adhesive layer. In FIG. 2, a layer structure made of the six layers is illustrated as an example, but the layer structure in the present invention is not limited to this, and any number and layer structure may be used.

[0032] The sensor element 101 is manufactured, for example, by laminating ceramic green sheets corresponding to each layer after carrying out predetermined processing and printing a circuit pattern thereon, and then firing the laminated sheets to integrate them.

[0033] The sensor element 101 is in the form of a long plate, and has six surfaces: two end surfaces in the longitudinal direction (a leading end surface 102a and a rear end surface 102b), two main surfaces along the longitudinal direction (an upper surface 102c and a lower surface 102d), and two side surfaces along the longitudinal direction (a left surface 102e and a right surface 102f). The left surface 102e and the right surface 102f are not shown in FIG. 2.

[0034] A measurement gas flow space 15 is formed on one longitudinal end side of the sensor element 101. The end on the side where the measurement gas flow space 15 is formed is hereinafter referred to as the tip. In this embodiment, a gas inlet 10 opens on the tip surface 102a of the sensor element, and the measurement gas is introduced from the gas inlet 10 into the measurement gas flow space 15 inside the sensor element 101.

[0035] Specifically, a gas inlet 10 is formed at the longitudinal tip 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 section, is formed in such a manner that a first diffusion rate-controlling section 11, a buffer space 12, a second diffusion rate-controlling section 13, a first internal space 20, a third diffusion rate-controlling section 30, a second internal space 40, a fourth diffusion rate-controlling section 60, and a third internal space 61 are connected in this order in the longitudinal direction from the gas inlet 10.

[0036] The gas inlet 10, the buffer space 12, the first internal void 20, the second internal void 40, and the third internal void 61 are spaces inside the sensor element 101, which are defined by an upper portion cut out of the spacer layer 5, the lower portion cut out by the upper surface of the first solid electrolyte layer 4, and the sides cut out by the side surfaces of the spacer layer 5.

[0037] 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 in the 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 each have any shape that provides a desired diffusion resistance, and the shape is not limited to the slits.

[0038] 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.

[0039] Further, 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, a reference gas introduction space 43 is provided 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.

[0040] The air introducing layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introducing layer 48 through the reference gas introducing space 43. In addition, the air introducing layer 48 is formed so as to cover the reference electrode 42.

[0041] 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 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 be in 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 ZrO2).

[0042] In the measurement gas flow space 15, the gas inlet 10 is a portion that opens to the outside space, and the measurement gas is introduced into the sensor element 101 from the outside space through the gas inlet 10.

[0043] 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 does not need to have a recess for the gas inlet 10. In this case, the first diffusion rate-controlling part 11 essentially serves as the gas inlet.

[0044] Also, for example, the measurement gas flow space 15 may be configured so that the measurement gas is introduced through a porous body.

[0045] The first diffusion rate-controlling portion 11 is a portion that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

[0046] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling portion 11 to the second diffusion rate-controlling portion 13 .

[0047] The second diffusion rate-controlling portion 13 is a portion that imparts a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0048] 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. In other words, 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 rate-controlling section 13. For example, it is also possible to adopt an embodiment in which the first diffusion rate-controlling section 11 directly communicates with the first internal space 20, that is, the buffer space 12 and the second diffusion rate-controlling section 13 do not exist.

[0049] The buffer space 12 is a space provided to reduce the influence of pressure fluctuations on the detection value when the pressure of the gas to be measured fluctuates.

[0050] 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 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 20 become almost negligible.

[0051] 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 rate-controlling part 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0052] 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 in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6 so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0053] The inner main pump electrode 22 is disposed on the inner surface of the measurement gas flow space 15 so as to face the first inner 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 partition the first inner 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 inner 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 walls of the first inner space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are disposed in a tunnel-shaped structure at the positions where the side electrode portions are disposed.

[0054] 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 containing 1% Au and ZrO2). 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.

[0055] 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 by 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 in the first internal space 20 out to the external space, or to pump oxygen from the external space into the first internal space 20.

[0056] 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.

[0057] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force (voltage V0) in the main pump control oxygen partial pressure detection sensor cell 80. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 so that the voltage V0 is constant. This makes it possible to maintain the oxygen concentration in the first internal space 20 at a predetermined constant value.

[0058] The third diffusion rate-controlling section 30 is a section that applies 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.

[0059] 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 part 30. The oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50.

[0060] 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 oxygen partial pressure of the measurement gas introduced through the third diffusion rate-controlling section 30 is further adjusted by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high accuracy, thereby enabling the gas sensor 100 to measure the NOx concentration with high accuracy.

[0061] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell including an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided over 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 in 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 on the inner surface of the measurement target gas flow space 15 at a position farther from the one end (tip) in the longitudinal direction of the base portion 102 (sensor element 101) than the inner main pump electrode 22 is.

[0063] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to 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 providing the ceiling surface of the second internal space 40, a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 providing the bottom surface of the second internal space 40, and side electrode portions (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 providing the side walls of the second internal space 40, respectively, forming a tunnel-shaped structure.

[0064] Like the inner main pump electrode 22, the auxiliary pump electrode 51 is also made of a material with a reduced ability to reduce the NOx component in the measurement gas.

[0065] In the auxiliary pump cell 50, by applying a desired pump voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 by 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.

[0066] 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 constituted 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.

[0067] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere in the second internal space 40 to be controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0068] In addition, the pump current Ip1 is used to control the voltage V0 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 the voltage V0 is controlled so that the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-controlling part 30 into the second internal space 40 is always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is kept at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0069] The fourth diffusion rate-controlling section 60 is a section that applies a predetermined diffusion resistance to the measurement gas, the oxygen concentration (oxygen partial pressure) of which has been controlled to be even lower in the second internal space 40 by the operation of the auxiliary pump cell 50, and guides the measurement gas to the third internal space 61.

[0070] 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 portion 60. The measurement pump cell 41 operates to measure the NOx concentration.

[0071] The measurement pump cell 41 measures the NOx concentration in the measurement gas in the third internal space 61. The measurement pump cell includes an inner measurement electrode (measurement electrode 44 in this embodiment) disposed in the measurement gas flow space 15 (on the inner surface of the measurement gas flow space 15), and an outer measurement electrode (outer pump electrode 23 in this embodiment) disposed at a position of the base portion 102 different from the measurement gas flow space 15 and corresponding to the inner measurement electrode.

[0072] That is, in this embodiment, the measurement pump cell 41 is an electrochemical pump cell configured by 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 a suitable electrode at a position other than within the measurement gas flow space 15, for example, outside the sensor element 101, will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0073] The measurement electrode 44 is disposed on the inner surface of the measurement gas flow space 15, at a position farther from the one end (tip) in the longitudinal direction of the base portion 102 (sensor element 101) than the inner main pump electrode 22 and the auxiliary pump electrode 51.

[0074] 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. For example, in this embodiment, the measurement electrode 44 is a porous cermet electrode of Pt, Rh, and ZrO2.

[0075] In the measurement pump cell 41, oxygen produced by decomposition of nitrogen oxides in the atmosphere surrounding the measurement electrode 44 is pumped out, and the amount of oxygen produced can be detected as a pump current Ip2.

[0076] In order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, is configured by the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0077] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 in the third internal space 61 through the fourth diffusion rate-controlling part 60 under the condition that 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. The generated oxygen is then pumped by the measurement pump cell 41, and the pump voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 is kept constant. The amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the measurement gas, so that the pump current Ip2 in the measurement pump cell 41 is used to calculate the nitrogen oxide concentration in the measurement gas.

[0078] 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.

[0079] 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 (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measured gas outside the sensor.

[0080] In the gas sensor 100 having such a configuration, the main pump cell 21 and the auxiliary pump cell 50 are operated to supply a measurement gas in which the oxygen partial pressure is always kept at a constant low value (a value that has substantially no effect on the measurement of NOx) to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be known 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.

[0081] Furthermore, in order to increase the oxygen ion conductivity of the solid electrolyte, the sensor element 101 includes a heater section 70 that adjusts the temperature by heating and keeping the sensor element 101 warm. 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 relief hole 75.

[0082] 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 the outside.

[0083] 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 a 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, and heats and keeps warm the solid electrolyte that forms the sensor element 101.

[0084] 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 areas to the same temperature, and the sensor element 101 may have a temperature distribution. For example, the sensor element 101 may be heated so that the temperature of the solid electrolyte and each electrode around the measurement target gas flow space 15 becomes about 750°C to 900°C.

[0085] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 102. 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 allows the above-mentioned main pump cell 21, auxiliary pump cell 50, and measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 102 as in this embodiment. Alternatively, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and disposed adjacent to the base portion 102.

[0086] 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 the heater lead 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 the heater lead 76, and between the third substrate layer 3 and the heater 72 and the heater lead 76.

[0087] The pressure relief hole 75 is formed to penetrate the third substrate layer 3 and to communicate between the heater insulating layer 74 and the reference gas introduction space 43. The pressure relief 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 relief hole 75 may be omitted.

[0088] Also, a predetermined length range from the tip of the surface of the sensor element 101 in the longitudinal direction may be covered with a porous protective layer (not shown). The porous protective layer is formed to protect the internal space and the region where the electrodes are present of the sensor element 101 from thermal shock caused by splashing water or the like. The porous protective layer is made of ceramics such as alumina, and may have a thickness of about 10 μm to 2000 μm. Also, it is preferable that the porous protective layer is formed so as to be able to withstand a force of up to about 50 N.

[0089] The above-mentioned sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the measured gas and the rear end of the sensor element 101 contacts the reference gas. The front end of the sensor element 101 is accommodated in the internal space of the element protective cover 120 as shown in FIG.

[0090] [Use of gas sensors in the presence of oxygen and hydrogen] Consider a case where a measurement target gas (in this embodiment, NOx) in a measurement target gas in which oxygen and hydrogen coexist is measured using the above-described gas sensor 100. The measurement target gas may be exhaust gas from an internal combustion engine such as a hydrogen engine or a gasoline engine.

[0091] For example, in a hydrogen engine vehicle, an exhaust gas purification device including an oxidation catalyst is usually arranged in the exhaust pipe downstream of the hydrogen engine, and unburned hydrogen in the hydrogen engine is oxidized in the oxidation catalyst. However, before the oxidation catalyst warm-up is completed during cold start, hydrogen may be discharged downstream of the oxidation catalyst without being oxidized. Before the oxidation catalyst warm-up is completed during such cold start, hydrogen may reach the gas sensor 100 without being oxidized in the oxidation catalyst, even if the gas sensor 100 is installed and used downstream of the oxidation catalyst.

[0092] When the gas sensor 100 is used, the sensor element 101 is heated by the heater 72 as described above, and the tip of the sensor element 101 where the measurement gas flow space 15 is formed becomes hot (for example, about 800° C.). This hot part is housed in the inner protective cover 130 and arranged so as to be in contact with the measurement gas. The measurement gas that reaches the vicinity of the sensor element 101 becomes hot by being heated by the sensor element 101 in a high temperature state. Therefore, when oxygen and hydrogen coexist in the measurement gas, a reaction between oxygen and hydrogen (oxidation reaction of hydrogen) may occur. The reaction between oxygen and hydrogen (oxidation reaction of hydrogen) is an exothermic reaction. If a local oxidation reaction of hydrogen occurs near the sensor element 101, it is considered that thermal stress is generated in the sensor element 101 due to the reaction heat.

[0093] The tip of the sensor element 101 where the measurement gas flow space 15 is formed is heated to a high temperature by the heater 72, so it is considered that a rapid oxidation reaction of hydrogen is likely to occur particularly at the tip of the sensor element 101. If a rapid local oxidation reaction of hydrogen occurs near the sensor element 101, there is a concern that cracks may occur in the internal structure of the sensor element 101 depending on the magnitude of the thermal stress generated at that time.

[0094] [Protective cover] Next, the configuration of the protective cover in the present invention will be described in detail with reference to Fig. 1 and Fig. 3 to Fig. 8. As shown in Fig. 1, the gas sensor 100 of this embodiment includes a metallic element protective cover 120 that protects the tip side of the above-mentioned sensor element 101, i.e., the portion of the sensor element 101 that comes into contact with the measured gas. The element protective cover 120 has a plurality of through holes for allowing gas to flow therethrough, and is configured to supply the measured gas to the tip of the sensor element 101 through these through holes.

[0095] In this embodiment, the element protective cover 120 includes a bottomed cylindrical inner protective cover 130 that houses a part of the sensor element 101 including the one end (tip), and a bottomed cylindrical outer protective cover 140 that is disposed outside the inner protective cover 130 and houses the inner protective cover 130. In this embodiment, the inner protective cover 130 corresponds to the protective cover of the present invention, and the outer protective cover 140 corresponds to the outer cover of the present invention.

[0096] The inner protective cover 130 is a cylindrical metal part with a bottom that accommodates a part of the sensor element 101 including the one end (tip). The inner protective cover 130 is arranged so that the axial direction of the inner protective cover 130 coincides or almost coincides with the longitudinal direction of the sensor element 101. The inner protective cover 130 has a bottom on the tip side of the tip surface 101a of the sensor element 101. A space exists between the sensor element 101 and the inner protective cover 130, so that the measured gas can flow through the space. The inner protective cover 130 accommodates at least the part of the sensor element 101 that should be in contact with the measured gas. For example, it is preferable that the inner protective cover 130 accommodates at least the part of the sensor element 101 from the tip surface 101a having the gas inlet 10 to the rear end of the outer pump electrode 23 in the longitudinal direction of the sensor element 101. Alternatively, the inner protective cover 130 may accommodate the part of the sensor element 101 where the measured gas flow space 15 exists. The inner protective cover 130 may further accommodate the position of the reference electrode 42 or a part behind it. This may be appropriately determined within a range in which airtightness is maintained between the front end side of the sensor element 101 that contacts the gas to be measured and the rear end side that contacts the reference gas.

[0097] In this embodiment, the inner protective cover 130 includes a first member 131 and a second member 132. The first member 131 includes a cylindrical large diameter portion 131a that is fitted to the outer periphery of the tip side of the housing 151, a cylindrical small diameter portion 131b that is smaller in diameter than the large diameter portion 131a, and a step portion 131c that connects the large diameter portion 131a and the small diameter portion 131b. The second member 132 includes a cylindrical tubular portion 132a that is fitted to the small diameter portion 131b of the first member 131, and a tapered bottom portion 132b that continues to the tip side of the tubular portion 132a. The rear end of the tubular portion 132a is deformed into a U-shape, and a gas diffusion piece 132c is formed. The two main surfaces (upper surface 102c, lower surface 102d) and two side surfaces (left surface 102e and right surface 102f) of the sensor element 101 are surrounded by the side surfaces of the inner protective cover 130 formed by the small diameter portion 131b of the first member 131 and the cylindrical portion 132a of the second member 132. Note that the inner protective cover 130 may be formed by integrally forming the first member 131 and the second member 132.

[0098] The inner protective cover 130 has an inner gas inlet H2 and an inner gas outlet H3. The inner gas inlet H2 is a hole (e.g., a round hole) penetrating a side surface of the inner protective cover 130 surrounding the sensor element 101. That is, the inner gas inlet H2 is formed on the side surface at a position where the sensor element 101 is present in the axial direction of the inner protective cover 130 (the longitudinal direction of the sensor element 101). Specifically, the inner gas inlet H2 is formed on a circumference of the small diameter portion 131b of the first member 131 at a position on the rear end side of the second member 132.

[0099] The inner gas outlet H3 is a through hole formed in the bottom of the inner protective cover 130. The inner gas outlet H3 is formed in the bottommost part of the bottom 132b of the second member 132. The inner gas outlet H3 may be disposed further toward the tip side than the tip surface 101a of the sensor element 101. For example, the inner gas outlet H3 may be disposed in a tapered side part of the bottom 132b. In addition, the bottom 132b is not limited to a tapered shape such as a cone shape or a truncated cone shape, and may be flat or the like.

[0100] The outer protective cover 140 is disposed outside the inner protective cover 130 and is a cylindrical metal part with a bottom that houses the inner protective cover 130. The outer protective cover 140 is disposed so that the axial direction of the outer protective cover 140 coincides or nearly coincides with the axial direction of the inner protective cover 130. The outer protective cover 140 has a bottom on the tip side of the bottom 132b of the inner protective cover 130 (second member 132). A space exists between the inner protective cover 130 and the outer protective cover 140, allowing the measurement gas to flow. The outer protective cover 140 is composed of a cylindrical large diameter portion 140a that is fitted to the outer periphery of the large diameter portion 131a of the first member 131 of the inner protective cover 130 and the outer periphery of the housing 151, a cylindrical body portion 140b that extends from the large diameter portion 140a to the tip side, a tip portion 140c that is formed in a bottomed tubular shape with a smaller diameter than the body portion 140b and has a bottom on the tip side of the body portion 140b, and a step portion 140d that connects the body portion 140b and the tip portion 140c. The tip portion 140c is fitted to the cylindrical portion 132a of the second member 132 of the inner protective cover 130. This fitting portion separates the space between the body portion 140b of the outer protective cover 140 and the inner protective cover 130 from the space between the tip portion 140c of the outer protective cover 140 and the inner protective cover 130.

[0101] The outer protective cover 140 has an outer gas inlet H1 and an outer gas outlet H4. The outer gas inlet H1 is a through hole formed at a different position in the longitudinal direction of the sensor element 101 (axial direction of the outer protective cover 140) from the inner gas inlet H2 and the inner gas outlet H3 of the inner protective cover 130. A plurality of outer gas inlets H1 are formed on a circumference of the body 140b of the outer protective cover 140 at a position on the tip side of the gas diffusion piece 132c of the second member 132 of the inner protective cover 130. The outer gas outlet H4 is a through hole formed at a different position in the longitudinal direction of the sensor element 101 (axial direction of the outer protective cover 140) from the inner gas inlet H2 and the inner gas outlet H3 of the inner protective cover 130 and the outer gas inlet H1 of the outer protective cover 140. A plurality of outer gas outlets H4 are formed on the circumference of the side surface of the tip portion 140c of the outer protective cover 140 at positions closer to the tip side than the inner gas outlet H3 of the inner protective cover 130. Alternatively, the outer gas outlets H4 may be formed on the bottom surface of the tip portion 140c of the outer protective cover 140. In this case, it is preferable that the outer gas outlets H4 are disposed at positions different from the inner gas outlet H3 of the inner protective cover 130 when viewed in a plane including the bottom surface of the outer protective cover 140.

[0102] FIG. 3 is a schematic diagram showing the flow of the measurement gas in the element protective cover 120 when the gas sensor 100 is attached to a pipe such as an exhaust pipe of an automobile to measure the measurement gas such as exhaust gas. As an example of the case where the measurement gas flows from left to right in FIG. 3, the flow of the measurement gas is shown by arrows. The measurement gas flows into the outer protective cover 140 from the outer gas inlet H1 of the outer protective cover 140. The measurement gas that flows into the outer protective cover 140 is diffused by the gas diffusion piece 132c of the second member 132 of the inner protective cover 130 and flows backward. Then, the measurement gas flows into the inner protective cover 130 from the inner gas inlet H2 of the inner protective cover 130. The measurement gas that flows into the inner protective cover 130 flows toward the tip side along the sensor element 101 and reaches the vicinity of the tip surface 101a of the sensor element 101. A measurement gas is introduced into the sensor element 101 through the gas inlet 10 on the front end surface 101a, and the measurement of the target gas in the measurement gas is performed. The measurement gas is discharged from the vicinity of the front end surface 101a of the sensor element 101 to the outside of the element protective cover 120 through the inner gas outlet H3 of the inner protective cover 130 and the outer gas outlet H4 of the outer protective cover 140. The measurement gas also flows in from the upstream outer gas outlet H4 (for example, the outer gas outlet H4 located on the left side in FIG. 3) along the flow of the measurement gas in the piping. The measurement gas discharged from the inner gas outlet H3 of the inner protective cover 130 is smoothly discharged to the outside from the downstream outer gas outlet H4 (for example, the outer gas outlet H4 located on the right side in FIG. 3) by the flow of the measurement gas flowing in from the upstream outer gas outlet H4. Since there are multiple outer gas inlet H1, inner gas inlet H2, inner gas outlet H3 and outer gas outlet H4, the gas to be measured actually flows in and / or out from holes other than those indicated by the arrows, generally following the gas flow described above.

[0103] In the gas sensor of the present invention, the protective cover (inner protective cover 130 in this embodiment) has a gas inlet (inner gas inlet H2 in this embodiment) that penetrates the side of the protective cover surrounding the sensor element 101, and this gas inlet is positioned so that each of the parameters described below (length ratio La / Lb and gas oxidation ability α in the sensor element 101) is within a predetermined range.

[0104] A detailed description will be given with reference to Figs. 4 to 8. Figs. 4 and 5 are schematic diagrams for explaining the length La and the length index Lb. Fig. 4 is the same partial cross-sectional schematic diagram as Fig. 1. Fig. 5 is an enlarged cross-sectional schematic diagram along the line VV in Fig. 4. In Fig. 5, the outer protective cover 140 is omitted. Figs. 6 to 8 are schematic diagrams for explaining the surface area S and the volume V. Fig. 6 is the same partial cross-sectional schematic diagram as Fig. 1. Fig. 7 is an enlarged cross-sectional schematic diagram along the line VII-VII in Fig. 6. The VII-VII cross section is a cross section perpendicular to the longitudinal direction of the sensor element 101 at a position rearward of the tip of the sensor element 101 of the inner protective cover 130 (second member 132). Fig. 8 is a perspective view of the sensor element 101. The configuration of the electrodes and the like in the sensor element 101 is omitted from the illustration.

[0105] 4, the length of the sensor element 101 in the longitudinal direction from the one end (tip) of the sensor element 101 to the midpoint of the gas inlet (inner gas inlet H2) of the protective cover (inner protective cover 130) in the longitudinal direction of the sensor element 101 is represented as length La. The measurement gas that flows into the inner protective cover 130 from the inner gas inlet H2 flows toward the tip side along the sensor element 101 and reaches the vicinity of the tip surface 101a of the sensor element 101, as described above. The length La indicates the flow path length of the flow path of the measurement gas that flows in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101.

[0106] The cross section shown in FIG. 5 is a cross section perpendicular to the longitudinal direction of the sensor element 101 at the center position of the longitudinal direction of the sensor element 101 at the inner gas inlet H2 of the inner protective cover 130 (first member 131). In this embodiment, as shown in FIG. 5, six inner gas inlet ports H2 of the inner protective cover 130 are arranged evenly on a circumference, but the number of the inner gas inlet ports H2 and the positional relationship with the sensor element 101 are not limited to this. In addition, in the cross section of FIG. 5, the inner surface of the inner protective cover 130 is circular. The circular shape includes a circle, an ellipse, a circle with at least a part being distorted, and the like. In addition, at each position in the axial direction of the inner protective cover 130, each cross section may have the same shape as the cross section of FIG. 5, or may have a different shape. The inner protective cover 130 may be substantially cylindrical as in this embodiment within the range where the sensor element 101 exists in the axial direction. Alternatively, it may be tapered or inversely tapered, or may have a part with a different inner diameter. Alternatively, there may be polygonal portions such as triangles, rectangles, etc.

[0107] In a plane perpendicular to the longitudinal direction of the sensor element 101 including the midpoint of the longitudinal direction of the sensor element 101 of the gas inlet (inner gas inlet H2) of the protective cover (inner protective cover 130), i.e., in the plane of the paper of FIG. 5, the index of the length between the sensor element 101 and the circle defined by the inner surface of the protective cover (inner protective cover 130) will be described. First, the longest distance between each of the two main surfaces of the sensor element 101 and the circumference of the circle defined by the inner surface of the inner protective cover 130 (first member 131) facing each of the main surfaces is represented as length x1 and length x2, respectively. In addition, the longest distance between each of the two side surfaces of the sensor element 101 and the circumference of the circle facing each of the side surfaces is represented as length y1 and length y2, respectively.

[0108] That is, the length x1 indicates the longest distance between the upper surface 101c and the circumference (upper part in FIG. 5) of the circle (circle shown by a broken line in FIG. 5) defined by the upper surface 101c and the inner surface of the inner protective cover 130. Here, the longest distance means the longest of the lengths (distances) of the perpendicular lines when the perpendicular lines are extended from each position on the upper surface 101c to the circumference (same below). The length x2 indicates the longest distance between the lower surface 101d and the circumference (lower part in FIG. 5) of the circle (circle shown by a broken line in FIG. 5) defined by the inner surface of the inner protective cover 130 and the lower surface 101d. The length y1 indicates the longest distance between the left surface 101e and the circumference (left part in FIG. 5) of the circle (circle shown by a broken line in FIG. 5) defined by the inner surface of the inner protective cover 130 and the left surface 101e. Length y2 indicates the maximum distance between right surface 101f and the circumference (the right part in FIG. 5) of the circle defined by the inner surface of inner protective cover 130 (the circle shown by the dashed line in FIG. 5) facing right surface 101f.

[0109] Using the above lengths x1, x2, y1, and y2, an index of the length between the sensor element 101 and the circle defined by the inner surface of the inner protective cover 130 is calculated. Length index Lb=(x1+x2+y1+y2) / 4 The length index Lb is an index indicating the length of the gap between the sensor element 101 and the inner protective cover 130 at the cross section of the flow path of the measurement gas flowing in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101. It is generally considered to be an index indicating the size of the cross section of the flow path. The sensor element 101 is a long rectangular plate, and the inner protective cover 130 is generally cylindrical in the region of the length La in the longitudinal direction of the sensor element 101. Therefore, in this embodiment, it is considered that the size of the cross section of the flow path is approximately the same throughout the region of the length La.

[0110] In this embodiment, as shown in Fig. 5, the sensor element 101 is disposed at the center of a circle (indicated by a dashed line in Fig. 5) defined by the inner surface of the inner protective cover 130. In this case, the length x1 is equal to the length x2, and the length y1 is equal to the length y2. If this is expressed as x1 = x2 = x and y1 = y2 = y, the length index Lb is given by Length index Lb=(x+y) / 2 This is expressed as:

[0111] The ratio of the longitudinal length La of the sensor element 101 to the lateral length Lb of the sensor element 101 is represented as the length ratio La / Lb. The length ratio La / Lb is an index indicating the degree to which the measurement gas that has flowed into the inner protective cover 130 contacts the surface of the sensor element 101 or passes near the surface before reaching the tip surface 101a of the sensor element 101. It is considered that the larger the length ratio La / Lb, the more likely the measurement gas that has flowed into the inner protective cover 130 will contact at least one of the two main surfaces (upper surface 102c, lower surface 102d) and two side surfaces (left surface 102e and right surface 102f) of the sensor element 101.

[0112] The total surface area of ​​the two main surfaces (upper surface 102c, lower surface 102d) and two side surfaces (left surface 102e and right surface 102f) of sensor element 101 over the length La in the longitudinal direction of sensor element 101 is represented as surface area S. Surface area S is the surface area of ​​the region present in the shaded portion of sensor element 101 in FIG. 6. In other words, referring to FIG. 8, if the surface areas S of the portions of length La from tip surface 101a of upper surface 102c, lower surface 102d, left surface 102e and right surface 102f are represented as Sc, Sd, Se and Sf, respectively, surface area S is given by S = Sc + Sd + Se + Sf The surface area S is the surface area of ​​the sensor element 101 that the measurement gas can come into contact with in the flow path of the measurement gas flowing in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101.

[0113] 6 and 7, the volume of the space (the shaded space in FIGS. 6 and 7; this space is referred to as the measurement gas presence space) existing in the protective cover (inner protective cover 130) from the tip of the sensor element 101 to the length La in the longitudinal direction of the sensor element 101 is represented as volume V. Volume V indicates the total volume of the flow path of the measurement gas flowing in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101. In this embodiment, volume V is approximately the volume of a cylindrical shape of length La, the cross section of which is a circle defined by the inner surface of the side surface of the inner protective cover 130, minus the volume of the portion of length La of the plate-shaped sensor element 101.

[0114] The ratio of the surface area S to the volume V is represented as the ratio S / V. The ratio S / V indicates the ratio of the surface area of ​​the sensor element 101 with which the measured gas can come into contact with the volume of the measured gas present inside the inner protective cover 130 in the flow path of the measured gas flowing in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101. As described above, it is considered that the measured gas becomes hot near the surface of the high-temperature sensor element 101, and a reaction occurs between hydrogen and oxygen in the measured gas. Therefore, it is considered that the larger the surface area of ​​the sensor element 101 is relative to the volume of the measured gas, that is, the larger the ratio S / V is, the more hydrogen in the measured gas can react with oxygen on the surface of the sensor element 101. In other words, it is considered that the larger the ratio S / V is, the higher the oxidation ability of the gas on the surface of the sensor element 101 is. This ratio S / V is also called the oxidation ability α of the gas on the sensor element 101. α=S / V.

[0115] As described above, the sensor element 101 is at a high temperature, and when the measurement gas in which oxygen and hydrogen coexist comes into contact with the surface of the sensor element 101 or passes near the surface, the measurement gas is heated to a high temperature, and a rapid reaction between hydrogen and oxygen (oxidation reaction of hydrogen) occurs. When the measurement gas is heated to a high temperature near the two main surfaces (upper surface 102c, lower surface 102d) and two side surfaces (left surface 102e and right surface 102f) of the sensor element 101, a reaction between hydrogen and oxygen occurs near the two main surfaces and two side surfaces of the sensor element 101, and hydrogen in the measurement gas is consumed and the amount of unreacted hydrogen is reduced. Then, the measurement gas after the amount of unreacted hydrogen is reduced reaches the gas inlet 10 that opens on the tip surface 101a of the sensor element 101.

[0116] In the gas sensor of the present invention, The length ratio La / Lb of the length La to the length index Lb is 0.9 or more, and The ratio of surface area S to volume V is 0.15 mm -1That is all. In this range, even if a measurement gas in which oxygen and hydrogen coexist flows into the inner protective cover 130, the surface area of ​​the sensor element 101 for oxidizing hydrogen in the measurement gas that has flowed into the inner protective cover 130 can be secured, and hydrogen can be effectively oxidized on the two main surfaces and two side surfaces of the sensor element 101. It is believed that hydrogen can be oxidized evenly in the portion of the length La from the tip surface 101a of the two main surfaces and two side surfaces of the sensor element 101, and that local oxidation reactions of hydrogen can be suppressed. If local oxidation reactions of hydrogen are suppressed, it is possible to suppress the occurrence of large thermal stress locally. Therefore, even when a measurement gas in which oxygen and hydrogen coexist is measured, it is believed that the NOx concentration can be measured accurately without cracks occurring in the internal structure of the sensor element 101. Furthermore, there are no openings on the two main surfaces and two side surfaces of the sensor element 101, and it is believed that the structural strength is relatively high compared to the vicinity of the gas inlet 10 of the sensor element 101. Hydrogen in the measurement gas is effectively oxidized on the two main surfaces and two side surfaces of such a sensor element 101, so that the hydrogen concentration in the measurement gas that reaches the gas inlet 10 of the front end surface 101a of the sensor element 101 can be sufficiently reduced. This makes it possible to further suppress the oxidation reaction of hydrogen near the gas inlet 10 and further reduce the occurrence of thermal stress due to the reaction heat. It is therefore believed that the occurrence of cracks in the sensor element 101 can be further reduced.

[0117] As described above, the length ratio La / Lb is 0.9 or more. Alternatively, the length ratio La / Lb may be, for example, 1.0 or more, 1.2 or more, 1.5 or more, 2.0 or more, etc. The upper limit is not particularly limited and may be set appropriately in consideration of the usage environment and purpose of the gas sensor 100, and the performance required of the gas sensor 100, such as responsiveness, which will be described later, etc. For example, it may be 7 or less, 5 or less, etc.

[0118] As described above, the gas oxidation capacity α (= ratio S / V) of the sensor element 101 is 0.15 mm -1 Or, the oxidation capacity α (= ratio S / V) is, for example, 0.18 mm-1 Above 0.2mm -1 More than 0.25mm -1 The upper limit is not particularly limited and may be set appropriately in consideration of the environment and purpose of use of the gas sensor 100, and the performance required of the gas sensor 100, such as responsiveness, which will be described later. For example, 0.5 mm -1 Below, 0.4mm -1 The following may be possible.

[0119] Even if the length ratio La / Lb is 0.9 or more, if the oxidation capacity α (= ratio S / V) is extremely small, the surface area of ​​the sensor element 101 becomes extremely small relative to the volume of the measured gas, and there is a concern that hydrogen in the measured gas cannot be sufficiently oxidized. -1 Even if the length ratio La / Lb is above this value, if it is extremely small, the flow path of the measurement gas flowing in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101 is too short, and the measurement gas cannot sufficiently contact the surface of the sensor element 101 or pass near the surface, and there is a concern that hydrogen in the measurement gas cannot be sufficiently oxidized. Therefore, when the length ratio La / Lb is 0.9 or more and the oxidation ability α (= ratio S / V) is 0.15 mm -1 It is better to do this.

[0120] For example, when the gas sensor 100 is used to measure the exhaust gas of a hydrogen engine vehicle, when the concentration of the gas to be measured in the measurement gas changes from moment to moment, it is required that the concentration output value of the gas sensor 100 has good responsiveness to the change in NOx concentration in the measurement gas. For example, the shorter the time required from when the NOx concentration in the measurement gas flowing outside the gas sensor 100 changes until the pump current Ip2 flowing through the measurement pump cell 41 becomes a current value corresponding to the changed NOx concentration, the better the responsiveness. When the measurement gas with a changed NOx concentration flows into the element protection cover 120, reaches the gas inlet 10 of the sensor element 101, is introduced into the inside of the sensor element 101, and reaches the measurement electrode 44, the pump current Ip2 becomes a current value corresponding to the changed NOx concentration. It is considered that the more quickly the measurement gas in the element protection cover 120 is replaced, the better the responsiveness can be obtained.

[0121] As described above, the length La indicates the flow path length of the flow path of the measurement gas flowing in the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the tip surface 101a of the sensor element 101. If the flow path length is short, the measurement gas in the inner protective cover 130 tends to be replaced quickly. Although it may vary depending on the responsiveness required for the gas sensor 100 and the configuration of the gas sensor, the length La is preferably 10 mm or less, for example. It is considered that good responsiveness can be maintained within such a range. Alternatively, it may be, for example, 9 mm or less, 7 mm or less, 4 mm or less, etc. Regarding responsiveness, the lower limit of the length La is not particularly limited, but the length La should be determined taking into consideration the above-mentioned length ratio La / Lb and oxidation ability α (= ratio S / V).

[0122] Although the gas sensor 100 for detecting the NOx concentration in the measurement gas has been described above as an example of an embodiment of the present invention, the present invention is not limited to this embodiment. The present invention may include various forms of gas sensors as long as they can achieve the object of the present invention, that is, to suppress the occurrence of cracks in the sensor element and accurately measure the measurement target gas in the measurement gas, even when a gas containing coexisting gas species that can cause an exothermic reaction, such as a gas containing coexisting oxygen and hydrogen, is introduced into the gas sensor as the measurement gas.

[0123] In the above-described embodiment, the gas sensor 100 detects the NOx concentration in the measurement gas, but the measurement gas is not limited to NOx. The measurement gas may be, for example, oxygen O2 or other oxide gas other than NOx (for example, carbon dioxide CO2, water H2O, etc.). Alternatively, it may be a non-oxide gas such as ammonia NH3. When the measurement gas is a non-oxide gas, the non-oxide gas is converted to an oxide gas (for example, in the case of ammonia NH3, it is converted to NO), and the measurement gas containing the converted oxide gas is introduced into the third internal space 61. The converted oxide gas in the measurement gas is reduced in the measurement electrode 44 to generate oxygen. The generated oxygen can be obtained as a pump current Ip2 in the measurement pump cell 41 to detect the measurement gas. The conversion of the non-oxide gas to an oxide gas can be performed by at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51 functioning as a catalyst.

[0124] Also, the gas sensor may be capable of measuring a plurality of types of gases to be measured. For example, the gas sensor may detect the concentration of nitrogen oxides NOx when the gas to be measured is in a lean atmosphere, and detect the concentration of ammonia NH3 when the gas to be measured is in a rich atmosphere. For example, the gas sensor may determine whether the gas to be measured is in a lean atmosphere or a rich atmosphere based on the electromotive force (voltage Vref) generated in the sensor cell 83, and detect the NOx concentration based on the pump current flowing through the measurement pump cell 41 as described above when the gas to be measured is in a lean atmosphere. Also, when the gas to be measured is in a rich atmosphere, the ammonia NH3 may be converted to NO in at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51, and the NH3 concentration may be detected based on the pump current flowing through the measurement pump cell 41. Such a gas sensor is effective when used downstream of a so-called three-way catalyst (TWC).

[0125] In the above embodiment, the element protective cover 120 is a double-structured cover including the inner protective cover 130 and the outer protective cover 140, but is not limited thereto. The inner protective cover 130 arranged at the position closest to the sensor element 101 may be configured to satisfy the length ratio La / Lb and the oxidation ability α (= ratio S / V) of the present invention. The position, shape, size, number, etc. of the inner gas inlet H2 and the inner gas outlet H3 of the inner protective cover 130 may be appropriately determined. For example, the shape of the inner gas inlet H2 may be a circle, an ellipse, a rectangle, or various other shapes. For example, a straightening piece may be formed near the through hole of the inner gas inlet H2. In addition, the presence or absence of the diffusion piece 132c of the inner protective cover 130 and the shape when formed may also be appropriately determined. The position, shape, size, number, etc. of the outer gas inlet and the outer gas outlet of the outer protective cover 140 may also be appropriately determined. Other shapes, plate thicknesses, etc. of the inner protective cover 130 and the outer protective cover 140 may also be determined as appropriate. The element protective cover 120 may be a single-layer structure (only the protective cover in the present invention), a triple-layer structure (for example, in addition to the protective cover and outer cover in the present invention, an intermediate cover), or a multi-layer structure with more than one layer. These may be determined as appropriate taking into consideration the usage environment of the gas sensor (flow rate of the gas to be measured, temperature, possibility of splashing water due to condensed water, etc.) and the performance required of the gas sensor, such as responsiveness, etc.

[0126] In the above-described embodiment, the sensor element 101 is formed by laminating oxygen ion conductive solid electrolyte layers. However, the portions not constituting the pump cells and the sensor cells do not have to be solid electrolyte layers and may be made of other ceramics such as alumina.

[0127] In the above embodiment, the sensor element 101 has an opening (gas inlet 10) on the front end surface 101a, and the measurement gas is introduced into the sensor element 101 from the opening on the front end surface 101a, but the present invention is not limited to this. For example, the measurement gas flow space 15 may be configured such that the gas is introduced from the vicinity of the front end of the side surface along the longitudinal direction of the sensor element 101 (base portion 102). For example, the side surface along the longitudinal direction of the base portion 102 may have an opening that communicates with the buffer space 12 or a position of the first internal space 20 near the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base portion 102 through the opening.

[0128] Even if the sensor element has an opening near the tip of the side, if the positional relationship between the sensor element and the inner protective cover 130 is configured to satisfy the length ratio La / Lb and ratio S / V of the present invention, it is believed that hydrogen can be oxidized evenly over the two main surfaces and two side surfaces of the sensor element 101 in the portion of length La from the tip surface 101a, as in the above embodiment, and localized oxidation reaction of hydrogen can be suppressed. Therefore, it is believed that the occurrence of cracks in the sensor element 101 can be reduced, as in the above embodiment.

[0129] In the gas sensor 100 according to the embodiment described above, the sensor element 101 has three internal cavities, 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 disposed in each internal cavities, as shown in Fig. 2, but is not limited thereto. For example, the sensor element 101 may have two internal cavities, 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.

[0130] In addition to the configuration of the internal space described above, each component of the sensor element 101, such as the measurement gas flow space 15 and each electrode, can take various forms depending on the type of gas to be measured, the purpose and environment of use of the gas sensor, etc.

[0131] The gas sensor 100 of the above embodiment is a so-called limiting current type gas sensor that detects the concentration of a gas to be measured based on the pump current Ip2 flowing through the measurement pump cell 41, but the gas sensor is not limited to this. It may be a so-called mixed potential type gas sensor that detects the concentration of a gas to be measured based on the potential difference between an electrode in contact with the gas to be measured and a reference electrode in contact with the reference gas. EXAMPLES

[0132] Examples in which gas sensors were specifically fabricated and tested will be described below as examples, although the present invention is not limited to the following examples.

[0133] [Examples 1 to 5 and Comparative Examples 1 to 3] For the gas sensor 100 shown in FIG. 1 and FIG. 2, the length La [mm], the length ratio La / Lb, and the gas oxidation capacity α of the sensor element 101 are expressed as α=S / V [mm -1 Gas sensors having different parameters for [1] to [3] were fabricated as Examples 1 to 5 and Comparative Examples 1 to 3.

[0134] The values ​​of the parameters in Examples 1 to 5 and Comparative Examples 1 to 3 were as follows. In Example 1, La = 7.1 mm, La / Lb = 3.3, α = 0.3 mm -1 . In Example 2, La = 4 mm, La / Lb = 3.3, α = 0.3 mm -1 . In Example 3, La = 10 mm, La / Lb = 2.3, α = 0.2 mm -1 . In Example 4, La = 10 mm, La / Lb = 0.9, α = 0.15 mm -1 . In Example 5, La = 14 mm, La / Lb = 3.3, α = 0.3 mm-1 . In Comparative Example 1, La = 0.6 mm, La / Lb = 0.2, α = 0.1 mm -1 . In Comparative Example 2, La = 10 mm, La / Lb = 0.6, α = 0.15 mm -1 . In Comparative Example 3, La = 10 mm, La / Lb = 0.9, α = 0.1 mm -1 .

[0135] In the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the position of the inner gas inlet H2 of the inner protective cover 130 in the longitudinal direction of the sensor element and the inner diameter of the inner protective cover 130 were changed to produce the desired length ratio La / Lb, the gas oxidation ability α=S / V in the sensor element 101, and the length La. In all of the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the dimensions other than the position of the inner gas inlet H2 of the inner protective cover 130 in the longitudinal direction of the sensor element and the inner diameter of the inner protective cover 130 were the same. In all of the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the same sensor element 101 and outer protective cover 140 were used.

[0136] [Crack risk assessment] The gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3 were evaluated for the likelihood of cracks occurring in the sensor element 101 (crack risk). Specifically, the gas sensor 100 was first attached to a test pipe and driven. That is, the heater 72 was energized to heat the sensor element 101, and the pump cells 21, 50, and 41 and the sensor cells 80, 81, 82, and 83 were operated as described above. The driving temperature of the sensor element 101 was set to 840°C. In this state, a model gas containing oxygen and hydrogen was flowed through the test pipe, and the gas sensor 100 in the driven state was exposed to the model gas for 100 seconds, and it was confirmed whether or not a crack occurred in the sensor element 101. The composition of the model gas was oxygen O2=5%, hydrogen H2=1.8%, and the remainder nitrogen N2. Note that all gas concentrations are expressed by volume (the same applies below).

[0137] Twelve gas sensors were prepared for each of Examples 1 to 5 and Comparative Examples 1 to 3, and the probability of crack occurrence in the sensor element (=number of cracks / number of tests×100) [%] was calculated from the number of cracks that occurred among the 12. The probability of crack occurrence was evaluated according to the following criteria.

[0138] A+: Crack occurrence probability is 5% or less A: Probability of crack occurrence is greater than 5% and less than 15% A-: Probability of crack occurrence is greater than 15% and less than 30% B: Probability of crack occurrence is greater than 30% and less than 50%

[0139] [Responsiveness evaluation] The gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3 were evaluated for their response in detecting the NOx concentration. Specifically, the gas sensor 100 was first attached to a test pipe and driven. That is, the heater 72 was energized to heat the sensor element 101, and the pump cells 21, 50, and 41 and the sensor cells 80, 81, 82, and 83 were operated as described above. The driving temperature of the sensor element 101 was set to 840°C. When the gas sensor 100 was driven, the pump current Ip2 flowing through the measurement pump cell 41 was in accordance with the NOx concentration in the measurement gas, as described above.

[0140] In this state, the first model gas with 0 ppm NOx (remainder is nitrogen N2) was flowed as the measurement gas in the test pipe, and the pump current Ip2 was waited until it stabilized. After the pump current Ip2 stabilized, the measurement gas flowing in the test pipe was switched to the second model gas with 500 ppm NOx (remainder is nitrogen N2). Then, the change in the pump current Ip2 over time was examined when the NOx concentration in the measurement gas was changed from 0 ppm to 500 ppm. The pump current Ip2 immediately before the change in the NOx concentration in the measurement gas (immediately before switching from the first model gas to the second model gas) was 0%, and the pump current Ip2 when the pump current Ip2 stabilized after the change in the NOx concentration in the measurement gas was 100%, and the elapsed time from when the value of the pump current Ip2 exceeded 10% to when it exceeded 90% was taken as the response time [msec] of the NOx concentration detection. The shorter this response time, the higher the response of the NOx concentration detection in the sensor element 101.

[0141] The above-mentioned response time was calculated for each of the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3. The response time was evaluated according to the following criteria.

[0142] A: Response time is 1200msec or less B: Response time is longer than 1200msec and less than 2000msec C: Response time is longer than 2000msec

[0143] Table 1 shows the parameters and evaluation results for the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3. Table 1 shows the values ​​of the parameters (length La, length ratio La / Lb, and gas oxidation ability α=S / V in sensor element 101) for each gas sensor, as well as the crack risk evaluation results, responsiveness evaluation results, and overall evaluation. The overall evaluation is based on the crack risk evaluation results, taking into account the responsiveness evaluation results.

[0144] [Table 1]

[0145] It was confirmed that the gas sensors of Examples 1 to 5 all had a reduced risk of cracking compared to the gas sensors of Comparative Examples 1 to 3. It was confirmed that the larger the length ratio La / Lb and the gas oxidation ability α=S / V of the sensor element 101, the more the risk of cracking can be reduced.

[0146] It was confirmed that the shorter the length La, the better the responsiveness of the gas sensors in Examples 1 to 5 and Comparative Examples 1 to 3. The gas sensor in Example 5 had the longest length La, and was slightly slower in responsiveness than the gas sensors in Examples 1 to 4 and Comparative Examples 1 to 3. It is considered that the longer length La of Example 5 made the time required for replacing the gas in the element protection cover 120 slightly longer than that of the other gas sensors. When better responsiveness is required in light of the purpose and environment of use of the gas sensor 100, it would be advisable to set the length La to, for example, 10 mm or less.

[0147] An overall evaluation was performed based on the crack risk evaluation results and taking into account the response evaluation results. Any gas sensor can be used in practice, but if oxidation and hydrogen can coexist in the measured gas, it is better to use a gas sensor with an overall evaluation of B or higher. Also, using a gas sensor with an overall evaluation of A- or higher is more preferable, as it can achieve both reduced crack risk and good response.

[0148] As described above, according to the present invention, even when a gas containing coexisting gas species that can cause an exothermic reaction, such as a gas containing coexisting oxygen and hydrogen, is introduced into a gas sensor as a measured gas, the occurrence of cracks in the sensor element can be suppressed, and the target gas in the measured gas can be accurately measured. [Explanation of symbols]

[0149] 10 Gas inlet 12 Buffer space 15 Measurement gas flow space 20 1st internal void 40 Second internal void 61 3rd internal void 100 Gas Sensor 101 Sensor element 101a (sensor element) tip surface 101b (sensor element) rear end surface 101c (sensor element) top surface 101d (Sensor element) bottom surface 101e (sensor element) left side 101f (sensor element) right side 120 Element protection cover 130 Inner protective cover 131 (Inner protective cover) first member 132 (inner protective cover) second member 132c (second part of inner protective cover) gas diffusion piece 140 Outer protective cover 150 Element encapsulation 151 Housing 152 Ceramic Supporter 160 Fixed part H1 Outer gas inlet H2 Inner Gas Inlet H3 Inner gas outlet H4 Outer gas outlet

Claims

1. a sensor element having a long plate shape including an oxygen ion conductive solid electrolyte layer, the sensor element having a measurement gas flow space formed on one end side in a longitudinal direction of the sensor element; a bottomed cylindrical protective cover that accommodates a portion of the sensor element including the one end, the protective cover has a gas inlet penetrating a side surface of the protective cover surrounding the sensor element, The length of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the gas inlet of the protective cover in the longitudinal direction is represented as length La, In a plane perpendicular to the longitudinal direction of the sensor element and including the longitudinal midpoint of the gas inlet of the protective cover, the longest distances between each of the two main surfaces of the sensor element and a circumference of a circle defined by the inner surface of the protective cover that faces each of the main surfaces are defined as length x1 and length x2, and the longest distances between each of the two side surfaces of the sensor element and a circumference of the circle that faces each of the side surfaces are defined as length y1 and length y2, respectively. When an index of the length between the sensor element and the circle defined by the inner surface of the protective cover is expressed as length index Lb=(x1+x2+y1+y2) / 4, A ratio La / Lb of the length La to the length index Lb is 0.9 or more, and When the total surface area of ​​each of the two main surfaces and the two side surfaces of the sensor element in the portion of the length La in the longitudinal direction is represented as a surface area S, and the volume of the space existing in the portion of the length La in the longitudinal direction in the protective cover is represented as a volume V, the ratio S / V of the surface area S to the volume V is 0.15 mm -1 The gas sensor for detecting a measurement target gas in a measurement gas is as described above.

2. 2. The gas sensor according to claim 1, wherein the length La in the longitudinal direction is 10 mm or less.

3. 2. The gas sensor according to claim 1, wherein the lengths x1 and x2 satisfy x1=x2=x, the lengths y1 and y2 satisfy y1=y2=y, and the length index Lb is expressed as Lb=(x+y) / 2.

4. The gas sensor according to claim 1 , wherein the protective cover has a gas outlet at a bottom thereof.

5. An outer cover for accommodating the protective cover is disposed on the outside of the protective cover, 2. The gas sensor according to claim 1, wherein the outer cover has an outer gas inlet at a position different from the gas inlet of the protective cover in the longitudinal direction of the sensor element.

6. 6. The gas sensor according to claim 5, wherein the outer cover has an outer gas flow outlet at a position different from the gas inlet of the protective cover and the outer gas inlet of the outer cover in the longitudinal direction of the sensor element.

7. 2. The gas sensor according to claim 1, wherein the measurement gas flow space of the sensor element has a gas inlet opening at an end face of the one end in the longitudinal direction.

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